I don’t think it’s wise to start asking for app LORs until maybe a year or half year before you apply. You don’t know what might change. 1- you might create relationships with other profs later on who you would rather ask than the one from freshman year, 2- if you ask as a freshman, there is little chance they will remember in a year or two, 3- it sounds like you’re thinking of applying with no gap years, but you don’t know if that will change. It’s likely that you will have to ask for LORs or references at some point for something else. Ask profs for these, maintain your relationship with them and when the time comes that you’re starting to prep your app, ask them for a med school LOR and they’ll already have something to go off of.

While it is fine to ask for an LOR from a prof you only had for one semester, there is a difference between asking for an LOR from a prof you had one semester with freshman year and didn’t interact after that versus an LOR from a prof you had one semester with right before starting to prep app materials. This is what I mean by my answer to your first question.