The Mascot is defeated, but Player A must defeat (but not necessarily kill) the wandering monster before being awarded levels or treasure (or the win).
Basically, the rules state that after you kick open a door, you give a "reasonable time" to allow other players to intervene.
In the FAQ, the following Q/A answers your question:
Q. Does the "reasonable time rule" apply to defeating a monster
without killing it, or just to killing it?
A. Any sort of defeat. If
you defeat it without killing it, other players still have a
reasonable time to play an applicable card to frustrate you. However,
if you removed the only monster from the fight (with Magic Lamp, for
instance), they cannot play Monster Enhancers or other similar cards
that affect a specific monster, because there isn't one there to
fight, and they can't use any special rules for bringing in monsters
(such as the Shark rules from Munchkin Booty) that depend on having a
monster in the fight. They can play Wandering Monster to bring in an
entirely new monster, though, but they must do it at once.
Basically, any ability that will auto-defeat the door mob still grants your opponents a "reasonable time" to respond, in which they can Wander in any monster they choose, which you have to fight just the additional monster.
Per this Super Moderator in the MunchkinForums:
Automatically defeating a Monster removes the Monster. Cards that rely
on a Monster can't be played, but that doesn't mean that combat is
over UNLESS the card that automatically defeated the Monster says
otherwise. This is a situation where cards could be played, but not
those that affected a non-existent Monster.
As far as why your other outcome couldn't happen, "Player A goes up to Level 10 and then gets killed":
Per the Munchkin Zombies ruleset:
- You cannot collect rewards for defeating a monster (e.g., Treasure, levels) in the middle of a combat. You must finish the fight before gaining any rewards.
And finally, this gets spelled out in the FAQ, Important Note #3:
"Killed" means that you defeated the monster with your Level and
bonuses in combat, or used some card or special ability that
specifically states that it kills the monster. For this, you get
levels and Treasure. Some Monsters say that they can be defeated
immediately, based on circumstances. Unless the card says otherwise,
this usually means an instant kill of the Monster, so you would get
levels and Treasure in those situations, too. However, even in a case
where a single monster is instantly defeated, other players get a
chance to play Wandering Monster to add a new foe that you must defeat
before claiming the spoils.
TLDR; defeating (or auto-defeating) a monster does not end combat; the monster is removed, but other players have a "reasonable time" to play any such as wandering monster--if done at once.
Should all additional monsters be defeated (or lamped, etc.), combat is then resolved and you can claim victory with the kill granted by your auto-defeated monster.