1. If you go from 500mb you'll get throttled. If you have the $20 UNLIMITED data plan you will get locked out of tethering at 500mb, but your phone data still works and won't slow down.
2. You can use up to the maximum you paid for.
3. Yes since you don't have the $20 unlimited data plan and got the $10 2GB data plan. You will slow down at 2.5GB.
4. You can use ALL of it. The $20 unlimited data plan is it's own animal and is separated.
5. They would pay $10 per 2GB up to 10GB I believe. Mobile data isn't meant to replace landlines.
Only the unlimited data plan you will not get throttled on your phone, but they will lock you out of tethering after you met the number you paid for (2.5GB you stop at 2.5GB etc.) tethering. You can keep using your phone at full speed.