Ok, the questions! (Sorry, I had to lay down yesterday. I wasn't feeling well after the shift overload, hehehe).
The laser gun does base damage of 3d6+3, plus another 1 every 3 full levels Icosa has...so right now that's a total of 3d6+4 damage. (Assuming Icosa is now level 5; see the last question below)
YUP!
I
'm not sure what the 'allowing for +5 aim' means. Does that mean it doesn't use Icosa's Proficiency bonus or attribute, but instead has it's own static bonus to hit?
Yup! When it shoots, it uses its own proficiency bonus. You can use yours if you so choose when you decide to employ the gun - and it agrees with you. Otherwise... Well, wait and see.
It seems that it defaults to an 'unready' configuration, hidden in Icosa's arm. What sort of action is required to make it ready to fire? A normal 'interaction' or a bonus action or full action or...what?
Simple interaction, except that you gotta roll against Cosemo's will, if Cosemo disagrees. If Icosa succeeds, the gun's ready.
Is firing it an Attack action? Can it benefit from having multiple attacks per Attack action, or is it more like casting a spell?
Yes, and yes. Also, if Cosemo decides to go rogue, it pulls from its own action pool. I'm not gonna tell you how large it is.
Does it have a damage type? What is its maximum range, or range increment?
Piercing, as the other guns. At first... As... the other guns...
Presumably the Detect spells it can cast are Cast Spell actions to use, yes?
Yes.
And finally; Icosa was a level 6 character when this item was gained. You asked if I was willing to 'give up' a level. Does that mean Icosa is now level 5? i'm unclear on how this works right now.
I was referring to your new multiclass choice. You do not have to relinquish a level... For now. (Just kidding; no need.)
That being said, [MENTION=4936]Shayuri[/MENTION], you do know you got a big share of the pie, but you're in all kinds of trouble, right? You now have yourself, your Patron and this thing contending for your actions!
What do I roll for survival?
Your lazy DM has built a dungeon table and you gotta roll against it to reach an appropriate exit. By the time you roll enough, the group finds a sweet spot to dig down, a passage or some other way to move towards the city. Until then... DUNGEON EXTRAVAGANZA! No, just kidding. But the 'roll to find the exit' is real. I think it'll make it vague just in the right way, and it'll allow me to cut boring transitions from the posts as well.
