I’ll check the challenges, but it’s possible there aren’t enough in the game right now.
For Lem, you can do both of them, it’s not an either or choice, instead its in case you don’t want to do one you don’t have to. However, you don’t get the opportunity to “go all the way” with him there in Ivald. Once you return to the city, Lem and Gemma will regularly appear in the inn. Lem can appear at either afternoon or evening everyday, one or the other. Once there, you can talk to him. Depending on how your choices in Ivald went, you may be able to get enough reputation together to invite him upstairs. If so, you can have non-penetrative sex with him a few times, then eventually penetrative sex.
If you exhaust all his dialogue (including his reaction to you being naked) and don’t get the option to go upstairs with him, you’ll have to wait until the next version. There are limited amounts of reputation you can earn outside of quests. That said, you aren’t missing much, I plan to implement NPC personalities into the encounter system, but I haven’t yet, so sex with Lem is just a normal NPC sex encounter right now. That’s why I really only left juuuust enough rep opportunities to barely unlock it, since its not that impressive yet.