Good question ;) Here's my $0.02:

I tend to feel that if I offer my advice and it's not taken, on your own head be it. I'm happy to offer it any time I'm asked, and would try to sneak it in if I see my friend in need, but if it's not wanted, I'll lose interest. This makes situations 1, 2, and 4 pretty easy calls for me.

For situations 3 and 5, I would probably get a more insistent, since there's a direct effect on me (I'll add selfishness to my confession later *grin*). Especially if it could adversely affect my job performance or continued employment. I don't think I'd take over entirely, but I would make sure my opinions are well-documented to cover myself if it all hits the fan.

The final situation is the easiest of all. I'd move ;P I don't really trust anyone to program a nuclear reactor control program, so I don't want to be anywhere nearby when it blows.

-rattus, rambling along

__________
He seemed like such a nice guy to his neighbors / Kept to himself and never bothered them with favors
- Jefferson Airplane, "Assassin"