If you simply don't use webmail, you're about 99% less likely to accidentally type your password into a website that happens to look like your webmail login page (which doesn't exist).
Sure. FWIW i avoid using webmail as much as i can, but that's just because i love my mu4e. Still though, i wonder about what would happen in the (admittedly very unlikely) event that someone compromised my secrets file. (example off the top of my head: seized backup disk at border control)
Yeah, depends on what your threat model is. You'll go crazy treating every possible threat as "guaranteed to happen; must defend" though. If you're worried about border control, at least you'll know if they search your stuff and can revoke access after the fact, which puts it in a slightly different category than day to day surreptitious password theft.