Hi Powerade_993,
1. Generally they give about 90 days for a termed vendor to catch up with the old A/R. But since you have a cut off date, the provider may even have a different arrangement (may be a claim basis) with the vendor until his All of his collectibles are brought in.
2. Yes. This is possible. A provider can submit claims via more than 1 clearinghouse. You just have to make sure you apply for a new EDI enrollment and not an update.
3. This is a bit complicated though, but since you will be taking charge beginning from a particular service date, you should report to the other vendor if you receive any payments for her claims and make sure you have a track of it. Similarly, she should send your payments to you from payers who have not updated the new address, so that both of you can reconcile your deposits accurately.
Hope this helps. Good luck with the new client.