I think your paste wax should be fine. If ever natuyo yon, malamang nagtutong lang yung top layer. Pero I doubt.

1. It should be safe to use as often as you like.
2. I don't know if paste waxes have expiration dates. But I do have a 1/4 full can of Soft99 carnauba paste wax which is already 3 years old. I got that from Concorde...sigurado old stock na yon... so I reckon at least 5-6 years old na! Heheheh... the wax is still okay. If ever may expiry period yan, you'd probably used up all the wax before it happens.