WE find the Epcot area resorts to be hot for walking around. They seem more exposed somehow to the heat and the wide concrete walkways are hot. OTOH, you have Storm Along Bay at the YC/BC which is nice if you plan to spend some down time swimming.
Personally, we prefer the MK resorts to Epcot resorts when it is hot, especially the Poly with its foliage or the CR which has everything conveniently included under one roof. The down side to the CR is the pool is plain and in the open. The GF is nice, but has wide concrete walkways that always seem hot in the heat to me. Fortunately, the GF is fairly small and easy to get around.
Queues waiting for boats is often (but not always) in the sun, waiting for the monorail or buses at deluxe resorts is under shade, FYI.
My opinion, FWIW:
I would probably do a split stay if staying 6 days or longer between the Poly and the YC.
The CR is closer to the MK, but the convenience of walking to the MK is not so great if you are trying to stay out of the heat. The Poly has the boat and monorail access to the MK.
The BC is closer to Epcot than the YC, but if you want to use the boats, the YC back door exits directly across from the boat dock.
I would focus on visiting the MK while at the Poly and then Epcot and DHS while at YC. You can choose when to go to AK from either resort. OR do a three way split stay and stay at the AKL for a couple of nights. It is really cool, everything is under one roof and is only a 5 minute bus ride from the AK. This is a great idea if you are staying 10 nights or more, IMO. ( 10 nights = 3-4 nights at the Poly, 4-5 nights at YC and 2 nights at the AKL. Perfect trip, IMO.)
Don't worry! May is warm, but there are hotter months (like August, ugh!). If you plan for the heat, it will be fine! Remember to take lots of swim/rest breaks
Enjoy!