It sounds like you found a pretty good interim therapist for a specific purpose, to get you through the issues with your therapist's illness and uncertain relationship. I see that as a benefit right there. With some time and distance, you can consider seeing the other therapist and do some research to find out more about her once you get her last name. It sounds like this interim therapist would be a good one to bounce that idea off of. That way, if you ever end up going to the other new therapist, you've been upfront with the interim one and it won't be as awkward.

Also, if there's a chance your therapist does return, you will not have started up with someone who works for her and then have complications from that.

My two cents.