Scenario: Investigator attacks the AO and her Combat Check results in 2 successes. If she exhausts her Marksman Skill to re-roll, do the 2 successes from the first roll still count? Thus, (with 4 investigators) if the re-roll resulted in 2 more successes, would a Doom Token be removed?

My guess would be that successes on the first roll are negated, with the second check completely replacing the first check...

you have to re-roll your successes too. when there's an exception to that rule, it's specifically stated (like on Mandy's power).

Right. Using Marksman can possibly make your roll worse.

Marksman is at its best when battling monsters, because you've either rolled enough successes to kill it, or haven't. It's clear when to use Marksman against a monster. Against an AO, however, you have to recognize when you'll be more likely to get more successes than you just got (or perhaps it's a last-ditch, do-or-die attack).

Cool. That's what I figured, but it's nice to get some clarification from other players. Thanks!