can a 1000 rated player defeat an IM

??

Yes: 1 game per 2 million
https://wismuth.com/elo/calculator.html#rating1=1000&rating2=2400 

Nope

Even with queen odds, NO.

#7
I once witnessed a case where an old lady drew against an IM in a swiss open tournament. The old lady made odd and bad moves, the IM took it lightly and blundered. In the end the IM could barely draw.

An IM or GM can throw a game. Lucky opponent.

Well, of course it’s “possible”… 

Yes it does. The OP did not restrict the type of game played.

     It's not possible for an actual 1000 Elo player to beat an IM in a normal rated otb game, but it is always possible for an IM to take such a game too lightly and make a devastating blunder while not paying attention. Although the IM might well be able to pull out the win anyway in such a situation, I have seen players in such a situation "blow up", resign and go off in a huff cursing their stupidity.

     On chess.com it's possible that a strong player neglected to choose an appropriate beginning rating, so such a player with the default 1200 rating could actually be strong enough to beat an IM.

There's less than 1% chance if they play their average in theory.

But in practice the 1000 rated player might be underrated. Happens all the time. I was 1400 6 months ago and I would beat 2100s from time to time but that's because I was underrated. 

Ask yourself the question: How does a 1000 rated player get paired with the IM? Probably because he's doing very well in the tournament

Is the 1000 a GM doing a speed run or ratings climb?

I once beat a WFM in FOG OF WAR

I mean, yes, technically it's possible...

In bullet, it would be easier, I've beaten a non-titled 2600 with a previous account in 1 min games, but my best win in blitz is a 2100, and in rapid an 1850.

I defeated an IM, so if a 1000 rated player could defeat me, then theoretically you could beat the IM I beat.