Doesn't matter whether the mops are floating or sinking. Some eggs are bound to stick in them eventually. The eggs are ridiculously adhesive. If you want to hatch the eggs that remain stuck on everywhere else, then you'll have to remove the parents rather than waste your time scraping each egg out. Most parents prefer to spawn during the early dawn.
Making spawning mops is very simple. Get a ball of yarn and make a pompon out of it. I once threw a ball of yarn in my goldfish tank since I was getting rather lazy and several eggs were stuck there.
A 3-inch goldfish should be able to release about 700-800 eggs. Don't expect all eggs to survive. The less fry you raise, the better for you. You will not have plenty of space anyway to raise all of them.
Check this link how to sex goldfish. I prefer to use the venting method but males should have breeding tubercles on their gill covers and pectoral fins. http://thegab.org/Goldfish/sexing-goldfish.html
The mops with eggs are easier but if you want to hatch those that remained stuck on glass, then moving the parents will be much easier. It's your choice though. You can leave the eggs to be eaten by their parents but if they have to eat more than they can handle, then do them a favor by scraping and disposing the extra eggs before they can foul the water very badly. Goldfish eggs stink badly when left to rot. You would be surprised the ammonia can spike and even kill the parents quickly.
No, as frequently as possible. The presence of protein loaded foods and appropriate environmental conditions will permit them to spawn even every few weeks but be careful what you wish for. Remember you cannot handle thousands of fry. Better have a few dozens of fry than thousands of them.