When you say you bolused for 15 carbs for a half an apple and "brown bread". A slice of regular bread is around 15 carbs; if it's cut thicker it is more like 20, so yes, 15 carbs for the whole meal sounds too low.
As for your questions: Yes, it sounds like you underbolused. So for corrections: it depends on your ISF. If you seek to correct at 2 hours you always have to check for IOB. So let's say you took 3 units; at 2 hours you still have one unit on board and if one unit drops you say 45 points, there would be no need to correct.
If you had taken more insulin would you have gone low in your 4th hour? You were 105 so if you took more it might have lowered you to let's say 95 or 85, neither of which are too low. As for Apidra, yes it works a bit faster and has a bit less tail. I consider my duration of action for Apidra to be 3 hours, so yes, that's a bit easier to handle if you are more insulin sensitive. But it's only a minor difference, and I don't see that you would have had a problem if you had taken a bit more.