Question of gym etiquette

Yes for sure , first come first serve , he came first let him finish and put his monster then you take your turn
Shaving also is a really annoying and Impolite way even if the monster you gonna put is a better defender

Unless you are going to help prestige to the next spot for him.... this is basically "sniping lite"

Sure it's bad but I think kids don't care at all. Not talking by my own experience but if I were 14-20 again I wouldn't care on doing something like that.

What's more maybe I'd think it's a perfect chance to leave my mons without struggle.

It's okay to join in and help but make your own spot.

Snipe it and give them the "don't hate the player, hate the game" look.

Yeah that is also bad etiquette IMO.

People have done that to me, I couldn't care less if that happens to them either.