The taunt has the same resistance as the Dead Ringer: 90%. With Pyro health that means you need to, essentially, deal 1750 damage to kill him during the taunt vs 1250 for a dead ringer spy.
Only because most spies are crap at using it. The trick, with the spy-cicle and dead ringer combo is that you walk around with your primary out. If you get caught by a pyro you dead ringer to put yourself out and cloak, then switch to the knife so that if they do manage to hit you again, it still does no good. You don't ignite and you're 100% invisible. If you're not a crappy spy and don't follow a predictable route there's not way for them to find you.
The Pomson is a nice sidegrade. It can do wonders at stopping a push if you can get the medic before he activates it, since each shot drains 10% uber.
The widowmaker is, easily, the best engineer primary around. Unless your aim is awful, it's essentially a shotgun that doesn't need reloading, has infinite ammo AND refills your metal. Since you still get metal from it when you hit disguised spies, it's also a method of spy checking. I've been using the regular shotgun lately just out of nostalgia's sake (since I prefer the alternatives on every class), but the widowmaker absolutely beats it, hands down, in the vast majority of situations.