Yes, FSRs do fail and are high on the troubleshooting list for a battery drain issue.
In case you need to replace FSR:
http://www.xoutpost.com/articles/x5/...nt-bmw-x5.html
Electrical issues are a pain and I think the best way to find a solution is to be sure you are eliminating a potential cause before proceeding with other possible causes. While fuses can be a conclusive test there be other associated fuses. I prefer to disconnect the suspected part so I know it can't draw current whenever I can fairly easily.
Fuse 64 (50A) is for heating and air conditioning. Related fuses are 3 (5A) and 48 (7.5A)
Symptoms of bad FSR can be fan not running, changing speeds, running intermittently or running after key is off, I would expect to find other symptoms indicating a bas FSR.
There are a myriad of other possible causes. Some of the ones that have more frequently been the cause are the phone which you can disconnect. (Noting you had a phone issue--does it tie to when this problem started?). The radiator coolant fan which you should be able to hear. The CD unit or the nav unit which you can disconnect. Also, check all heat/air/fan controls to see if they are working properly.
I would also verify that the testing of the battery included a load test or do one on your own. Auto parts stores will often load test the battery free of charge.