Applying the FMJ is more of a preference thing whether or not you want to use a buffer or apply it by hand. Personally, I usually apply last step products by hand. I find it easier to spread thin and you are more accurate getting in tighter areas of the vehicle and it's faster for me.
If you do use a buffer to apply it, use a speed setting of 2 or 3 and a blue (or black) pad.
Regarding the cure time, if you top it right away, you're going to sacrifice the durability of the FMJ. If the coat hasn't fully cured and bonded to the paint, when you wax over it you'd be primarily left with just wax on your paint and little to no sealant protection. If you weren't planning on waiting, there's almost no benefit in doing both steps other than spent time and product.
I always recommend waiting for the first maintenance wash to top it with a coat of wax. This also gives you time to appreciate the look of the sealant and compare the differences after the wax is on there.
Hope this helps,