I've never been able to quantify any tangible benefits from more than two applications of sealant. I haven't been able to measure any discernable increase in thickness, nor have I experienced additional durability.

Some people say that a new application of product "re-awakens" the sealant already on the paint, and only succeeds in re-spreading the previously applied sealant thus negating any idea of adding an extra layer as perceived. I guess it depends on whether the cured sealant is strong enough to withstand emulsification chemicals designed to suspend the now cured polymers.

Some might, but not any that I'm explicitly aware of. I should try ten coats of Duragloss 111 and see what happens...

In the past "wax buildup" was something enthusiasts wanted to avoid because it tended to "cloud" the finish.