On point #3, you can set the outgoing caller id to the user's phone number after you verify it by posting to the OutgoingCallerIds List Resource: http://www.twilio.com/docs/api/2010-04-01/rest/outgoing-call...

There is a validation that has to be done with the number to make sure you own it, but you should be able to use it as much as you want after that first validation.