Used to be a chomper main in GW1 but didn't played him much in the sequel,I'll try to do my best to help anyway:
1) Chomper probably have the best variety for his upgrade, the only bad upgrade for him is speed right now (which is afaik,bugged anyway). Everything else is a good build.
2) I don't have disco chomper unlocked so I can't help at that.
3) Stick to the stock one, Super sticky goop cooldown is too long, same for chomper cannon in top of...Not helping that much. Sprint burrow is bad and bugged, Spiky spikeweed damage both is also not worth giving up Basic spikeweed amount and superior bulk. But unlike the rest,it actually usable.
4) Lazy to find anything right now, @ECM_Atlas actually got most of the good tip for chomper already anyway.
5)Fire and power are good breath variant while armor is a excellent biting variant. The rest are decent expect for toxic and chomp thing which are bad.