Is my DM abusing/breaking rules?

So to give a bit of context since my DnD session will be starting here later tonight me, and a few of the party members of our current DnD Group think our DM is breaking rules or just being completely unfair to the entire party expect for two. 

We're using 5e small bit of home-brew but very small bits, story started in a place called Ember-watch, Potions are very rare with it costing a lot of platinum (Lesser Healer being 10 while Strongest Healing is 100,000), and of course no gods to follow/worship because either most were killed off or imprisoned.

Our party (Who are all Lv 6)
(This is also suppose to be a Hard-Core Campaign) 
Me - Dragonborn Warlock/Sorcerer 
Grac - Half Orc Fighter
Mcgee - Tiefling Rouge
Sam - Dragonborn Druid/Rouge 
Paarthurnax - Human Cleric
Shadowlily - Gnome Bloodhunter (Who is also running a Glass Cannon Build)

Now Mcgee is majorly always busy or due to time zones rarely with the group which we normally understand but the recent encounter our DM did was so unfair but if Mcgee was with us our DM told us it would of been harder since we faced 40 Bandits, 6 Mages, 1 Arc Mage, 2 Bandit Leaders, and previously hired Merc (Who dipped on us turn one when we did hire him) turned into Bandit Leader. The Merc had many scrolls majorly being Counter Spell (He had 15 of them on him), and another scroll that turned at least 10 of those Bandits into a  Dragon Turtle the Mages would cast invisibility on the Merc n Dragon Turtle while the rest spammed Fire Balls (Having at least 5 Magic Slots for each spell) while the Bandits were pretty weak the Dragon Turtle was the major issue with the Arc Mage. What made this encounter impossible though is that our DM spammed Counter Spells due to the 6 Mages getting three counter spells each turn so I was basically useless I could only use Melee or Command due to everything else counting as a "Spell" our Cleric couldn't do anything either including our Druid so it was majorly Melee Combat focus. 

We also have Relics in this Campaign but Grac our Fighter is the only person to have at least have two them one of Relics a Specter (Which reading its requirements needs a unresolved will to complete a mission even though it can be impossible) If it was powered basically made you almost immortal if you could get it to work, and another Relic called the Cinder-brand a Symbol that imbued itself to the holder which has a nasty debuff being your not allow to escape from fights since your body always thirst for combat. (It also has the benefits of when your attacked you add Cinder counters to your character so when you do attack you remove a counter to deal 1d4 Fire Damage) This Cinder-brand relic also has a final stand ability allowing the user to be at 15 HP but take 1d4 Fire Damage each time they end their turn, everyone is down, Dragon Turtle is weak, the rest of the Bandits (Expect for the Merc n Leaders who just dipped) are dead expect for 3 Mages left (Who do have weapons but our DM didn't use them at all since he majorly wanted to focus on spells) due to everything we made it through the encounter just to have Grac die at the end because he was so low of HP until our DM uses the Blind Old Man we've encounter a few times to basically res everyone back up. 

Me, The Cleric, and our Druid/Rouge couldn't do anything regardless if we reacted or not most of our attempt to help the party was null n void because we majorly got counter spelled I took up to the air to actually do something even with Breath Weapon, Cleric just buffed the party the best he could without getting counter spelled, and our Druid/Rouge already used up his transformation n couldn't sneak past anyone it was basically impossible for anyone else to do anything so we had to let the Melee focus classes do all the work.

Is my DM breaking any rules? Since it feels like most of the Spell Casters/Useful people are getting punished here while Grac/Mcgee get special treatment almost like favoritism.  

(Small Edit here) 
This isn't the first time our DM has spam counter spells majorly it was plot reasons for the most of them but for other reason it was just to stop the spell casters to stop spamming spells, and in other cases he just kept giving most enemies we've encounter either immunity or resistance to most of our spells. 

It’s debatable if they’re breaking any rules but they’re only got a casual relationship with the concept of encounter balance. Assuming the dragon turtle used the actual dragon turtle stat block that would be well beyond deadly as an encounter for level 6 players before you even add on the rest of the bandits, mages, and everyone else. Similarly just the archmage and the mages would take the encounter to deadly for that party. Spamming counter spell like that seems reasonable for that number of mages but definitely isn’t fun for the players bit would at least be using up all their reactions and be burning through their spell slots.

So I don’t think they’re cheating exactly but they’re not running a particularly fun or balanced encounter 

10 platinum for a basic 2d4 +2  potion. In other words, 100 gold for something which can restore 4-10 HP, less than a second level character has. I don't know why 1 million gold potions would even exist at all. No one is going to spend that much on a one-use item, which means no one would make any. Might as well just ban them completely and stop pretending.

One encounter with 40 enemies sounds super-deadly, if they only get one attack each, then on average, that's 2 crits on the party each round. Never mind all the tougher enemies on top. Even if they all have 1 HP each, that sounds like it would be hard to clear with AoE nukes. ...Oh, I just got to the part where you're getting counterspelled 3 times a round. Okay. Fighting 40 enemies in melee one at a time is at best insanely tedious.

This campaign doesn't sound like fun to me, and since you're questioning it, it doesn't sound like you're having fun either. Talk to your and other players DM about how un-fun that encounter was, and be prepared to walk away. No D&D is better than bad D&D.

All the Bandits had roughly 14 AC (The same AC I have on my Warlock) Arc Mage, and the Mages have 18 AC which wasn't so bad to deal with since our party kept hitting the AC count or above, and the Dragon Turtle for its AC if I remember had 22 AC it was basically an impossible battle we were just barley able to get by but the skin of our teeth. 

Right now the Cleric, Fighter, and Bloodhunter think these encounters are bullshit but the only person having fun in these encounters even though we're barley getting by is the Fighter because he likes cutting it close regardless if its impossible. Cleric hated how he could do nothing to heal the party unless all the mages used up their counter spells which only I was doing basically making them burn of all of them so we could heal (If we could at all), and Bloodhunter even agrees it feels like the DM favors the Fighter, and Rouge more than the rest of the party since no one else has relics expect for the Cleric who has an Artifact that basically buffs his holy damage with the benefit with keeping him at 1 HP until its his turn.

The only rule it seems the DM is breaking is that Polymorph can only turn people into Beasts, and a dragon turtle is, y'know, a dragon. 

That said, a dragon turtle is CR 17, an Archmage is CR 12, and a regular mage is CR 6. (This is to say nothing of the 40 bandits who can kill you all through sheer action economy alone) There is no way a regular party of level 6 PC's gets through even round 1 of that combat un-TPK'D without EXTENSIVE homebrew. With the power boosts you'd have to have been given to make it as far through the encounter as you did, it's impossible to say if the DM is being unfair, because they've basically already made you gods at level 6.

Last I can recall we did get a buff to keep everyone at 1 HP if we choose so but using up that buff entirely making it useless since it was a one time thing, Fighter has two relics (which one doesn't even work right now so its there as decoration), Cleric has an Artifact that buffs his holy damage with also having a buff to keep himself at 1 HP until its his turn, and for everyone else it was majorly due to us buying armor/weapons to help for the upcoming fights if there were any. 

I only have 14 AC for my Warlock I tried to get new armor but since my main focus of armor is Light I am forever stuck at 14 AC, Rouge has 16 AC, Fighter n Bloodhunter has 18 AC, Druid/Rouge has 20, and Cleric has 22 AC the only reason why we kept going down was due to Fire Ball spam n Arc Mage having nothing but AoE spells that hit our party which also did tons of damage. 

Arc Mage would roll almost up to 6D6's while the Dragon Turtle was throwing 10D6's we weren't really gods we were just given good items that meant nothing during this fight.
(Max Lv Cap is 20)

To answer your question about the Dragon Turtle yes it was using its stats too meaning we were fighting a impossible battle we only able to get by due to all the mages uses Fire Ball, and due to the AoE explosion damage was taking out their own guys we also kept living out the battle due to us helping escort mages to their city with the Mages selling their finest health potions to villages or stops all of the potions they sold before the encounter they were sitting with four creates worth of platinum all 10mil each. 

I believe our DM just wanted to wipe the party but was interested to see if we could even live.