Individual quest should not lock lona out from receiving other quests including guild quests. Only quest from guild will lock lona from taking another guild related quest. You can cancel guild quest by talking to the guy behind the counter in the same bar that posted guild quest.
No, I mean I want to get musket eventually, but apparently not just killing, but also sparing adam means I won't be able to progress in the quest line (see the guide - prerequisite quest won't spawn unless I start it before accepting the Milo's assasination job). So I'm just trying to figure out, what variables do I need to edit to roll back the quest.
Edit: so I'm becoming quite fast at finding what I need to find. seems that editing $story_stats["RecQuestMilo"] should be enough to reset it if you haven't interacted with Adam yet (not sure how to revive him).