Question will stay open approximately until spacex announces the date for flight 5.

It's your choice if you want to do so, but keeping the market open doesn't impact the profits/losses of people that have bet before the announcement.

Keeping the market open does have the benefit that we can go here to check what the current consensus is on the chances, even during the flight.

When is the cutoff between "not attempt" and "attempt but abort"? Like, if they say at the beginning of the launch they will attempt, but at the start of the boostback burn they decide there are too many issues to attempt and they never bring the booster near the landing site, is that an attempt or not?

beginning of the launch once the countdown starts.

If the booster enters inbetween the arms but fails to be caught and falls to the bottom of the pad and explodes, is that partial success or failure?

Good question. partial success if booster returns to the arms, even if arm and/or booster is significantly damaged.

Fail/abort if booster doesn't return to the arm.