1) If you go on 1v3 Meele ... then yes, it can be hard ...
Thats why every companion (and i believe even your Tav when you are alone at this point) tells you "better keep your/mine distance, one strike can be lethal" ...
Then it depends on how much you wish to cheese it ... it can be quite hard, but also very, VERY easy ... for example, you can dip your arrow in fire and blow up that illithid canister.

That will reduce them a lot.
But overall, if you fight from max range they rarely manage to get to you.
2) Never happened to me either.
3) The trick here is leting him escape ... all things Goblin have on them are marked as "owned" meaning you are not looting, you are stealing ... and NPCs react to stealing
You stole too soon > his escape routine was replaced by "expose a thief" routine.

Im not really sure its a bug, since this technicaly work as intended ... more like, unfortunate combination of consequences ... but maybe Larian could implement some kind of unmarking things as owned, when owner is dead.

Meanwile ... just give him enough time to get lost and it should be fine.

4) Have you used Friends cantrip in last few minutes?
