I like the vibe of waxed stuff but DIY is always an adventure and you'll need a lot of wax for that. Never heard of having a piece synthetically waterproofed aftermarket.
You should be looking for a fishtail that comes waterproof if you really need full waterproofing and demand it be a fishtail.
Ain't a problem, don't worry about it.
Timbaland, your dress shoes will be fine; put em in shoe bags inside a travel bag.
Birkenstock makes a back-strap sandal.