1 - I've seen chaosed Jinpachi attacking my team 3 times (got chaosed after getting an extra attack from himself or Sasori). Not sure if this is a (visual) bug or intended "feature".

2 - They never stacked with each other, only latest debuff applies. This also applies to 2 round and 1 round chaos - you can overwrite your Shisui's chaos with Kurenai etc. This one is not a bug.

Thats all good, Chaosing ninja that already has extra attack will cause him to attack his own team/field several time (Chaosing Ay4 with Shisui for example)

Chaos, Immobile and Sleep are all action impairing debuffs that are slightly diiffrent and they dont stack. It might seem weird as other debuffs can be placed at the same time - these 3 however all stop ninja from performing any action

Can you explain where Kushina gets 3rd attack from?

The first situation applies that way with every ninja under bee mystery and is the reason why it stacks with effects like pakura or boulder jutsu.

The second one is not a bug at all. Immobile, chaos and sleep replace each other once they are applied and is that way because, otherwise, effects that remove just 1 layer of debuff would be too weak otherwise.