1. Layering actually means "filling the spots you maybe missed". I would do it right the first time and later on add more protecting using the product as a drying aid. There was info about the curing time on this forum in the Opti-Seal thread:
2. You can but it will not last that well. I use it always as a drying aid after the ONR wash.
3. No-one can tell but probably every wax that don't contain cleaners and stuff that remove the sealant?
4. I think Opti-Seal sheets the wax pretty quickly but maybe you would have more of a wet look for a time?