Wash moves up behind Huron and says "No, no. Now let me help you out of that contraption, so we can go next door and speak."

 Wash, make a Persuasion check with advantage.

Persuasion: 23

[Shenanigans question: With a nat 20, can I convince him to let me get in the clockwork armour?]

(Shenanigans denied!  That nat 20 will get you exactly what you asked for, and nothing else.)

Oh, very well, but call your halfling friend back to watch the dragonborn.  I don't want any mess in my workshop.  Now, let's go to my chamber downstairs, we might get some peace there. 

Stahlmast jumps down onto the ground and begins to walk with Wash towards the stairs, giving Balassar a very unfriendly look as he goes.

[Hehe, fair enough, thought it was worth asking. I'm guessing only having light armour proficiency would probably mean Wash couldn't wear clockwork armour anyway.]

Wash walks behind Huron and when Huron has passed Balassar wash makes a hand gesture, pointing at Huron, then pointing down the stairs and then doing a stab-stab motion towards Huron and nods his head to ask 'got it now?'

Wash is trying to make sure he is between Huron and the clockwork armour going forward.

Balassar watches everything as his irritation grows. He then notices Wash make his movements and finally understands as he waits for the right Huron to get down the stairs before running him through on his blade.

New Round One:
Alton
Yinra
Wash
Stahlmast
Balassar

(OOC:  Literally everyone gets a turn before Balassar who initiated the resumption of hostilities.  I think that's RAW?)

Alton is up  Alton, you are near enough to the stairs to see Balassar and what he's doing. 

Alton moves next to Yinra helping to block the secret tunnel and gets his crossbow ready while saying quietly to her as he faces Huron's direction, "Get ready this is going to happen really fast." 

Movement: To the square just below Yinra
Action: Alton is going to hold off on firing his crossbow at Huron until after Balassar makes his attack.

Yinra nods and will also ready a longbow attack for when the first blow is struck in earnest.

Wash is up.

Wash says to Huron "So, friend, tell me more about this war you want us to help you with. What is our part going to be in it?"

Wash moves out of the way slightly (one square south if he can fit or one square north-west if not) and holds his action.

When Balassar attacks:

 Stahlmast sees the hostile intent, and turns to see Wash blocking the passage behind him.  These ruffians intend to murder me.  Let me past so I can finish them off. 

He takes no further action.

Balassar is up.

(Balassar's action will trigger lots of other actions too...)

Balassar just thrusts his scimitar forward at Huron's back.

OOC:

Attack: 21 Damage: Unable to parse dice roll.

That's a clumsy miss, but it does break the charmed effect from Stahlmast, and he realises that he has been ensorcelled.

Wash, you run towards the clockwork armour; as you do so, tries to hit you:

Attack: 22 Damage: 1

But without effect.  You don't reach the armour in time this turn.

Alton, you can also make the attack that you readied.  Plus it will be your turn in a moment anyway, so go ahead and make two attacks.  You have sneak attack from Balassar, but no advantage. 

Readied attack: As soon as he sees Balassar make his attack Alton aims and fires his crossbow at Huron. Attack: 9 Damage: 8 piercing

Turn: Alton says to Huron, "What is that large suit of armor for? Are you compensating for something?" and casts Vicious Mockery at him.

Movement: None
Action: Casts Vicious Mockery at Huron, DC 14 Wisdom save or take 1 psychic damage

Huron WIS save:  5

Huron seems to stumble for a moment, but is barely injured.

Yinra fires the longbow shot she had prepared:

Attack: 7 Damage: 7

New Round Two:
Alton
Yinra
Wash
Stahlmast
Balassar

Alton has already gone (above)

Yinra moves up the stairs and takes another shot:

Attack: 20 Damage: 8

This time, Huron Stahlmast stumbles more severely.

Wash is up.

Wash runs for the armour and hops in - or if he cant get in, blocks the part that Huron had been in previously using his body. [Does that take my movement and action? If not, I'll also angelic blast him.]

Wash, it does take your full movement and action to clamber into the clockwork armour, and once inside you realise quickly that you cannot operate it, and nor can you do much else.  You certainly are too restricted to cast any magic with a somatic component.  However, you have prevent Stahlmast from entering and using it.

I think Stahlmast didn't move before, so the map was wrong.  Stahlmast is now running towards the armour or so it seems, so Balassar you get an Opportunity Attack on him and he ends up, having dashed, at the point on the map below.

Balassar is up.  Take your Opportunity Attack from last time, and your regular action for this turn.

As Huron moves past Balassar swings his weapons at the man and watches his movement. He moves closer to him and breathes out his ice breath.

OOC:

Attack of Opportunity: Attack: 18 Damage: 10

Movement: 30ft diagonally

Attack: Breath weapon | Con save: 12 for half damage | 7 cold damage