So cover only cancels hits, which are already cancelled by armour and cover can't block critical hits. Am I missing something here?

Unless I'm mistaken, order of operations is that Cover and Dodge tokens activate first, removing hits rolled, then you "Modify Attack Dice" this is where Impact converts hits to crits, and then Armor means all remaining hits don't count.

Cover would cancel before impact. So if you had 2 hits, impact 2, cover 1. Cover 1 goes down to 1 hit which goes through via impact

Ah, that's what I get for not looking at the rules myself.

when did speeder bikes get the armor keyword? or am i missing context here?

OP is talking about air speeders (though he said snowspeeder) not speeder bikes. The rebel T-47 Airspeeder does have armor.