1. Yes, you can inject basal and bolus at the same time if you're going to eat but bear in mind the basal will bring your levels down a little, so take this in account when working out your bolus. The problem with taking both, or just bolus, before bed is that if you overshoot and end up over correcting, you run the risk of going hypo in your sleep with the potential dangers that this brings. If I want to snack at night because you fancy something or are hungry, then I tend to eat and bolus when I know I'll still be up for a while to keep an eye on levels. Alternatively I have the snack and take the high but usually I'm not prepared to do this, so if I am hungry at night, I go for a low carb snack and don't bolus, but everyone is different. Just be really aware of the risks of bedtime bolusing.
2. Same as above really. You can correct when needed, so not necessarily at mealtimes, but I tend to find this ends up with a bit of a hypo-high seesaw effect. If you regularly find you need to correct away from mealtimes, then I'd do a basal test to look at whether your basal level is right. This should be keeping your levels between meals fairly steady so corrections are not needed.
