If a follower is dismissed, do all of their equipment disappear? (other than their base equipment) Thanks.

No. From what I can tell, they retain it all. It doesn't look like it at first as their clothing appears to revert back to what they had on originally. But if they follow you again, it will change back (usually on a fast travel or location change).

No,they may wear the armor that they originally had but all the equipement you gave them is in there inventory.
When you ask them to follow you again they will equipe the stuff you gave them.