Last Comment Wins!

So Ini2 and Ini1 are terrified.

Ini1 is at 0 from your attacks.

order

Lair
Ini17 (no longer frightened) +60ft away (using +/- to indicate which direction)
Ini15a (still frightened), 1 4th level expended, +60ft away 
Ini15b (not frightened) -30ft away
Ini11 (still frightened) +60ft away
You
Ini2 (frightened) +60ft away

Who’s turn is it?

end of turn save for Ini2 (forgot)

11 or 2

Quote from TheFriendlyArchfey >>

Who’s turn is it?

 Lair action followed by Ini17

A pool of water (I’m assuming there are pools of water nearby) attempts to drag Ini15b towards me. DC 15 strength save or be pulled up to 20 feet towards me and be knocked prone.

Ini15b STR save: 10

Ini17 calls out, "Get ahold of yourselves, guys! We have a dragon to fight!" He too casts Polymorph (figure their best chance of winning is to try using up your resistances and then hope they can doll a bunch damage to you at once in a lesser form at once before you're knocked back to dragon; and then go from there)

Will you use a legendary action?

Side note, it shoudl 15b, not 15a with the used 4th level slot

Wisdom save: 26 or legendary resistance if its a failure.

*Im enjoying watching this*

Legendary action: all creatures within 15 feet of me must make a DC 23 dex save or take 10 bludgeoning damage. I then fly 40 feet to ini17.

Quote from King_Dragon_Slayer_SpiderKing >>

*Im enjoying watching this*

 *its really fun*

So you're at 2/3 legendary resistances
Also believe you've used to 3/3 legendary actions so Ini11 should go ahead without needing to wait

15a end-of-turn save (will be +120 ft from you now)

10 or 19

15b's DEX saving throw can't succeed (his highest is 22) so he's at 64/74

15b stands up, looks at the dragon next to his ally, and shouts, "GET READY TO CLOBBER!"
He casts polymorph once again. But not on the dragon this time, but Ini17. Smoke swirls around the Verdan sorcerer as it become a Bone Devil (reading the polymoprh spell, it goes with equal CR or level and Bone Devil is CR 9 so should be correct I believe.)

Ini11 end-of-turn save 
12 or 15

To double check
Lair
Ini17 0ft away, 1 4th expended, is Bone Devil
Ini15a +60ft away (frigtened) (since you got closer to 17 and he'd be the dash away) cast Ini17 as Bone Devil
Ini15b -50ft away
Ini11 +60ft away (tbd frightened based on results)
You (2/3 legendary resistances)
Ini2 0ft away (?, since hasn't dashed) (frightened)

So Ini11 is still frightened.
And I should note Ini15b as having used 2 4th level slots

Quote from TheFriendlyArchfey >>

Quote from King_Dragon_Slayer_SpiderKing >>

*Im enjoying watching this*

 *its really fun*

 *I can see*

*I would try and join in, but it’s getting late, and I wouldn’t want to spoil you two’s fun. Enjoy!

I exhale acid at the bone devil and all creatures in a 10 foot wide, 90 foot line behind them. Make a DC 22 dexterity saving throw, taking 65 acid damage or half a much on a successful save.

let's say you're hitting Ini15a Bone Devil and Ini11 (have been rather 2-D in describing distances prior so may as well get a second in line)

Ini15a Bone Devil Save: 18
Ini11 save: 11

Ini2 end of turn WIS save
6 or 14 will be +60ft away (opportunity attack if you want)

And then Lair Action

Insects swarm around ini2 in a 20 foot sphere. They must make. DC 15 con save or take 16 poison damage. At the end of every turn, if they’re in the cloud, they take the damage again.

Ini2 CON save: 15

Ini17 (mispoke earlier about 15a Bone Devil) rears up for a multi-attack
Claw 1: To-Hit: 12, Damage: 5 slashing
Claw 1: To-Hit: 11, Damage: 6 slashing
Stinger: To-Hit: 12, Damage: 11 + 14 poison damage (and roll DC 14 Con save or be poisoned)

Legendary actions?

 Tail attack: 33 damage: 17 bludgeoning.

Alrighty. 

Lair
Ini17 0ft away, 1 4th expended, is Bone Devil (69 HP polymorph)
Ini15a +120ft away (frightened TBD)  cast Ini17 as Bone Devil
Ini15b -50ft away  64/74 HP, 2 4th lv expended  (cast Ini17 as Bone Devil) //had this in the wrong place before
Ini11 +60ft away (tbd frightened based on results)
You (2/3 legendary resistances)
Ini2 0ft away (frightened) in fog cloud

Ini15a WIS save
15 or 19

Legendary actions? Or go to Ini15b?

Oh sheesh, I put the cast Ini17 as bone devil in the right place while leaving it in the wrong one too

Also, Ini11 should be at 9 HP

EDIT: ...and 120ft away

Edit: nope, because he hasn't had his turn yet