Lack of refrigerant in the AC will only pop the book symbol on an EARLY P38 - or a late one with the incorrect HEVAC fitted.
This is because early P38s drove the AC clutch directly from the HEVAC (through the safety pressure switches). It monitored the current drawn when the clutch was engaged. No gas would open the pressure switch thus cut off the clutch - raising a fault.
Later P38s changed to driving the clutch through a relay, because driving it directly was frankly a bit of a crap idea - corrosion in connections etc would often cause it to not engage properly. When they did this, the software in the HEVAC was changed to disable this fault detection - because a relay doesn't draw much current compared to the clutch, and this would have triggered the fault otherwise.
As for the condenser fans - ignore them if they are not running while you are diagnosing - on a standard P38, they will rarely come on in the UK climate IF your viscous engine fan is operating properly. They will only come on at low speed if the high side pressure reaches 17 bar, and switch to high speed (you'll know this when you hear it) at 21 bar. With a viscous fan even free-wheeling pulling air over the whole cooling pack, this basically never happens here.
If yours is a 2001 and has the correct/original to it HEVAC, no gas will not trigger a fault for the AC - it is blissfully ignorant of the state of charge and will try to engage the clutch all day long if it wants it, but the safety switch will do the job of preventing actual operation (rightfully so, don't run it with no gas or you'll be looking for a new compressor and flushing metal debris out of the system).
A dead blower - either because of a power supply fault (very common) or actual dead motor / brushes / controller will raise a fault though. Really... as I'm sure you already know, plugging it in with a nanocom or otherwise is the best place to start.
But it wouldn't be a bad idea to check RL6/7 in the fusebox - does the spot in the box look melted at all? After that, the connections in the drivers footwell - there is a plug with two thick wires in it. White green and white purple I think. These are the positive feeds from RL6 and 7 in the fuse box to each blower. Cut them out of their connections and crimp/solder them directly together - this causes a lot of blower stop faults due to corrosion in the connectors. Its probably more likely to be those or the fusebox (or a combo of both) than an actual dead blower in my experience.
The traces in the fusebox are marginal for these feeds - when you add in extra resistance from corroded connectors, its unsurprising how often this happens.
What will NOT cause a melted fusebox however is blocked pollen filters. A blocked filter = less air flow = less worked done by the motor = less current drawn by the motor. Just think about a vacuum cleaner - when you block the hose, the motor speeds up. Why? Because its got less load on it - so it spins faster using the same or less power as its doing less work :)