Let's attack each of these
Varlamov has another year left at $5m. Unless the Islanders are retaining on this trade, the Oilers can't fit him next year. I don't see the Islanders retaining on Varlamov as they will want to retool in the off-season considering all of their vets and long term deals.
Holtby is playing for Dallas who is currently battling Edmonton for a playoff spot. They are 1 point back with 1 game in hand. Unless something absolutely drastic happens, I can't see Dallas trading their starting goalie to the team they are battling down the stretch.
Greiss is an option. @ $3.6m, the Oilers would need to send Koskinen back to make it work which I am sure Detroit wouldn't mind. He's 36 years old and hasn't played more than 34 games in a season in 3 years. This year he is at 19 game played. He does nothing to solve their goaltending issues next year unless they want to run a tandem of Smith/Griess...
To me, Georgiev gives them someone to pair with Smith for the next couple of years, at a reasonable cap hit (the Rangers could retain half and take Koskinen back this year) and, based on his track record, probably get him locked in to a 2-3 year deal at around the same cap hit he is at now if desired. Based on their cap constraints, they really need a good, not great goalie signed at a reasonable cap hit so they can allocate those dollars elsewhere. This just makes so much sense, it's odd it hasn't happened yet.