Hi! Welcome to Flyertalk!

Your situation with Delta is not the best option - that's a lot of cash and miles. If you have them, spending miles for direct business class tickets is likely a better deal. It's 210K United miles for the two business class tickets, which is less than $5000. It might even be better to spend Delta miles, though I have no idea how many it would take thanks to their "flexible" system.

Since United is part of Star Alliance, you should look at www.staralliance.com to see the carriers, and flight paths, that go from Fort Myers to Warsaw. Then you'll have some ideas.
If you have more time, there are directions on how to use the ANA tool on Flyertalk; search for "ANA Masterclass". Using those tools, just glancing for you, there's some availability on LOT from JFK to WAW on April 25, and using USAir from PHL through FRA.

Since you don't get charged until you make the booking, my first option would be to call United and Delta and ask them to search. Once you have options, you can make a decision.