Planes seem to be much fuller these days...
Your concerns are valid. You could be delayed on either end.
Don't have anything urgent planned for a while after your hoped-for return date.
As long as you have no job, responsibilities and love to spend overnights in an airport you should have no problem. Sorry, the airlines no longer fly half empty as in the old days(5-10 years ago).
You might want to post this on the airline forum -- maybe more people with buddy pass experience there. Hard to say -- I do know my last several flights to the islands have been nearly full -- and the buddy pass gives you very low priority. I imagine something would be available within 2-3 days -- if you got the pass from a friend they should have access to check availability at the current time.
You'll be a nonrev and at the bottom of the list. A friend of mine got stuck on Maui for about a week before giving up and flying HA to HNL to have a better chance of getting on a flight to the mainland as a standby.
May is a slower month here, esp. during the middle when you are visiting. The 21st is a Monday, which could be okay but can be a busy day..............I'd go onto Alaska's site and check the flights for the 21st to 23rd or so and see what the load is. Keep checking it to get a feel for how full they are getting as it gets closer.
The red eye flights are the most popular, so if there are daytime flights you may stand a better chance of getting on one of those.
So there is a chance that you will be "stuck" on Maui, forced against your will to hope for a ride home? (dang...why didn't I get one of these tickets...never should have gotten reserved seats)
I know stranded on a tropical island...how awful :-)....I'm probably going to take my chances.