Thanks for the feedback! We are aware of the multiplayer bugs. Sadly it seems like we ended up pushing Playroom to its limits in some fields, and it's pretty clear that it was meant to be used for less complex games (also, making a multiplayer game for a jam is hard).
The boss's health is another issue that arises from this, since whether it's too large or too low depends entirely on the amount of players playing (since there is health scaling) AND on the abilities you got on that run. Generally, playing with 2-3 players should feel good, playing alone should feel like the boss has too much health, and playing in groups of 4 is pretty much unplayable past the first boss because of the lag.
And I'm sorry, I don't really understand your third point and I'd be glad if you could explain it to me. The objective of the trading mechanic is that you get random weapons so, if you want a new one, you talk with your friends to trade your weapon with theirs, and if you don't you can easily get it on another run. In my opinion, making it so each boss gives you a new weapon would make that side of the game pointless, but again, I'm not really sure that's what you meant. Thanks for playing!