Sitch 1: Man, that's about as good as it gets. If you'd gone anywhere in fair territory, you risked getting pegged. Not to mention the fact that you can lose aspects of the play by not keeping them in front of you. 3B foul line is as good as it gets.
Sitch 2: You screwed the pooch on this one (take that in a constructive tone, of course). Giving the signal or saying the word "infield..." is enough to constitute a "call." If you called it, you called it, and you have to deal with it. You were lucky this time, and I doubt you'll make the same mistake twice. It happens.
But hey... If this league is sticking you by yourself, then the players are going to have to accept the fact that despite your best efforts (which I believe you're making), you will miss calls by not being everywhere at once. If they want better coverage, then they need to pay for a second umpire.
Glad to see these kinds of questions, though. Shows you really want to work at it!