Doing a Campaign we are currently level 4 and am working on progression for a Witch Knight style.
I started Paladin 2 and have picked up Warlock 2 (Hexblade). Using Fiendish Vigor and Misty Visions as Invocations.
I plan to swap to Maddening Hex and Fiendish Vigor at 3.
I am a variant Human and got War Caster Feat. I am Unsure of Background to choose.

Warlock Cantrips: Eldritch Blast(w), Green-Flame Blade(w), 
Warlock 1st: Armor of Agathys, Hex, Shield,
Paladin 1st: Wrathful Smite, Divine Favor, Cure Wounds, Bless

The main focus and play for this character is to act as the "Bodyguard" for the main bruiser and support any VIP NPC's

My stats are Point buy at 13str 12con 14wis 16cha, I''m wondering if going to 15str at 6th level to put on plate for 20AC or to go the Magic Initiate Feat(Wizard) to pickup 
Find Familiar, Mind Sliver, Minor Illusion would be better choice for the playstyle. 

Endgoal is to get Warlock to 5 and then dump rest into Paladin. Thinking of picking up Thirsting Blade to help my damage until paladin's extra attack comes online and swapping out Fiendish Vigor for Improved Pact Weapon. Once Paladin extra attack comes online grabbing Eldritch Smite over Thirsting.
Spells I''m looking at picking up from Warlock levels are
Lightning Lure
Mirror Image
Summon Undead/Shadowspawn or Vampiric Touch

My guess is this campaign will be somewhere around the 7-12 range.

So in our last session I was next to our Barbarian/Druid and I was able to negate about 80+dmg over the fight by using the Intervention fighting style by prioritizing which enemy to block for on him. and once they targeted me I popped shield and used Armor of  Agathys while stabbing them with my hex Longsword. 

So currently using Fiendish Vigor and Misty Visions. at Third Level swapping Visions for Improved Pact, and then at 5th getting Maddening Hex to pseudo make up for not getting a second attack. I understand Thirsting would be better but I dont have levels to spare to go to 6 and rather not waste it. plus in tight grouped enemies it has better potential "niche" I know but on a budget. 

I grabbed Eldritch Blast because It's as you said decent and on par with other spells. and as I have Psychic and Fire with Mind Sliver and Green Flame, I was looking at making sure I had resistances options covered, which also led me to consider the Magic Initiate Feat(Wizard) for Toll the Bell's Necrotic. I considered getting repelling but the Barbarian doesn't seem to plan on getting any Sentinel/Reach combo options so forcing them to keep re-entering threat range isn't viable in this scenario.

The vast majority of Campaign at the moment is Culling Undead/Barbarian Tribes/Lizardfolk in Swamps surrounding a half flooded major city. so Upkeeping Fiendish Vigor is not an Issue. and alot of combat is in Ruins and small islands.

Reason I need to go Warlock first after getting 2 Paladin is because of the taking the Oath mechanic of Paladin, and needing to then stay Paladin or lose out on all the Conquest goodies.

The big struggle I have is figuring out Ability points,
currently I am thinking 14str+1 13wis 15+1cha and 10con/dex/int to wear plate with shield giving me 20AC

or ignoring plate and staying in Chainmail and having 13str 13wis 15+1cha 12con 10dex/int

In-Game world mechanics, once you take your oath as a paladin and go and level elsewhere you will lose your oath and become oathbreaker. however looking more at it, i am thinking Oathbreaker would be good choice over Conquest.

For a Background, I strongly recommend "Outlander" as one of the most useful ones for both supporting a party and being solo when running around the wilderness.

I also run a variant human Vengeance Hexblade Pallock and built it as 2 levels of paladin followed by 2 levels of warlock but only because doing so in that order was a result of events within the campaign story. And my group's DM limits a paladin's leveling based only on character behavior, not an artificial homebrew ruling. He doesn't automatically look at warlocks as "evil" but does require players to justify the combination of a paladin's oath with a warlock's pact agreement. Easy to do with some serious thought on the player's part.

I put more points into CON rather than WIS because, as a paladin, I'll constantly be in the front line. The extra hit points will be more consistently useful at all levels than an extra +1 WIS modifier. Besides, the paladin aura bonus to saves helps offset a low (or nonexistent) WIS modifier. I used the first ASIs from Paladin 4, Warlock 4 to buff my CHA from 14 to 18 and ended up having STR 13, DEX 14, CON 14, INT 09, WIS 12, CHA 18.

The starting feat I chose as a variant human was Magic Initiate (Warlock) to get Eldritch Blast, Mage Hand, and Expeditious Retreat (for battlefield maneuverability). My level 2 Warlock invocations were Agonizing Blast (good buff to Blast damage) and Devil Sight (defense against Darkness spells and dark environments in general). Yes, this means War Caster is delayed until my third ASI but something had to give and sticking my sword in the ground next to me while I launched a somatic spell was an acceptable workaround. If I happen to lose my sword (it's happened) there's always Eldritch (Agonizing) Blast to fall back on. No, he'll never compete with Fighter or Barbarian damage but he's the group tank/off-healer who can move extremely fast with Expeditious Retreat and then use Misty Step to teleport behind a target and scream in their ear to get and keep their attention.

In other words, he's the group "body guard". Just be sure to always keep one spell slot available for the Shield spell as a reaction. That thing turned a successful dragon's bite that would have killed my pallock into nothing but a loosened tooth for the dragon.