1) Normally on situations (cows) like this people don't skip (because they're not asked to, by opponent), so I asked. If it was him cowing this I'd say "np".
2) People can't just change their decisions in game whenever they want. If that's the case, everytime I play WXW and someone cows I'll say "dunno" or I wont answer, and I'll wait for the turn I'll fall near a barrel/piling my other worm/plop chance and then tell him to skip. It doesnt work like that - if you want me to skip, that will be in the turn it should, or something close to it - as in our case: when I lose 1 attack (1 worm) and not two.
3) You wouldn't wait another turn for me to skip, like I said, and you just left. Leaving is against the rules, leaving means lose by default. Bad luck.