9: I’d say, “don’t play fourth line stones if you can’t handle an invasion.” If your plan is to get invaded, force your opponent low, and take influence, a fourth line play near your opponent is fine. If you’re worried about the safety of your groups, go for third line. In general, you also don’t want all of your stones on the same line. All third line stones means you don’t have much in terms of territory, while all fourth line leaves you weak to invasion. In this case, your opponent has an obvious invasion point that will cut off your two fourth line stones, so I’d go for territory. (Also, in this particular situation, it’s tough to see a good use for influence, though that’s not always the case.)
51 (variation line): yes, that’s the infamous monkey jump , something you definitely have to watch out for with high, influence-based plays! getting hit with a monkey jump, or allowing your opponent space to play one, isn’t necessarily a bad thing, but it’s one of those situations where you want to know what you’re getting in exchange.
Also, I wouldn’t get too bogged down in the first 50 moves or so: that’s your strong point! Midgame is more of a weakness, so working midgame (direction of play and shape, mainly) will improve your game a lot more. (In my experience, working the opening was a lot easier, so I similarly had a lagging midgame for a while. Joseki and prop openings are easy to find and imitate, midgame resources are a bit more theoretical. Realistically, I still have a relatively bad midgame that I need to work on!)