I've done it with a 5 month old. the flight itself wasn't a big problem actually, as baby slept a lot of the way. we used a sky cot which worked well, only problem was turbulence so we had to disturb ds sleep to take him out of the sky cot. it was easier to take a 5 month old than a 1 year old which we also did as by 1 they do not want to sit still.
I can't comment on June heat with a baby as our trip was January, I do know august heat is brutal and with a baby, a good sun shade and lots of air conditioning are a must, which when you're looking for shelter and cool air is actually there in abundance.
there are so many rides at Disney that you can do with a baby and of course parent swap is available.
one of the nicest things is watching the characters interact with tiny babies, they love making a fuss and we got some gorgeous footage of ds babbling to daisy duck!
I would say go for it, bearing in mind when baby is hungry you need to stop for feeding, which can be more time consuming and frequent than you'd think, as long as you work the whole trip on total flexibility then you'll be fine
