Hi and welcome.
To save repetition, please see my post here with details on removing the Heater Motor Control Module.
C5 X7 Heater Motor Control Module
There can be 2 issues with these:
1) There is no fan speed on any setting, due to moisture in the bearings of the motor itself that can cause them to seize - and as these have a 'slow start' system typical of climate control systems, there isn't enough welly to get it going and free the bearings. So the solution is to short the pins to give it the full power and bypass the resistor - which others have done - until the next time it happens.
2) The second issue - which is more likely to be your problem is that the Blower control unit itself has gone - typical symptoms are all or nothing - which describes your problem So the resistance has gone basically. Most replace the module and that should sort it.
As for the battery disconnection - disconnecting and connecting the battery on these cars is akin to pulling the plug out the mains on your PC when it is running and without first shutting it down!
Modern multiplexed cars like the X7 have many data networks going through the vehicle linking up the various ECUs the the master computer or controller if you like - called the BSI. It allows the car to basically do more with less wiring and of course provides redundancy in the event of damage to one of the circuits.
So when you unlock the car in the morning it wakes up the various data networks on the understanding you are about to go for a drive. Similarly, when you switch the engine off and lock the car, the car takes a few minutes to ensure all the data stored in the ECUs is memorised. If you have an OEM navigation system, it will allow the units fan to cool the unit before switching off, the EGR butterfly valve will move to its default position etc. So that's why it is important to allow all the ECUs to go into sleep or standby mode before you disconnect the battery. And on reconnection, allow a few minutes for the ECUs systems to reinitialise before starting the engine.
Any questions you have - if the topics don't already exist, then do create a new one - but do keep the issues in separate threads please, as it can get confusing for others.
) it’s 08:54 as of posting, so would of got back to you shortly, but glad you have sorted it now in time for the drive!