We read the other threads but most of them appeared to have other identified issues contributing to their problem like broken keys, fuses, low bat, etc. We aren't aware of anything broken (but we are VERY non-technical). Opened the car door using key FOB, car did not start, horn started, and red light flashing fast etc.

Stopped the horn using Key FOB, closed door with plan to reopen after the red light slowed down, but as soon as we unlocked, the red light went nuts again and the whole mess started again. 2001 D2 and this is the first time it has done this. Your advice is very appreciated!

 

First thing to do is to look and see if all 5 locks are locking and fully unlocking. If they are then try locking and unlocking the door 3 times slowly, this can clear some alarm issues.

 

We tried the "lock and unlock" several times to no avail and then the car went dead..couldn't even power the windows. AAA just arrived and they think it is a bad cell in the battery so are replacing the battery. Land Rover wants us to tow it in saying it could be the alternator. I suppose it could be but it is a three year old battery. Curious why when my kid left a light on in the car and discharged the battery it did not cause this issue with the alarm system? Does the bad battery make sense as the cause if the alarm issue?

 

maybe a stupid question but did you try unlocking the door with the key in the drivers door?

A bad battery can have some strange effects on these alarms. The only way to disarm the alarm in a disco 2 is to unlock the doors there is no actual disarm wire. my suggestion would be to unlock the doors by pulsing the unlock wire at the BCU under the passenger side dash, the unlock wire is blue/grey and is at pin #22 in a 22 pin plug.
I would be very careful if you try this as the wrong wire could cause damage to the vehicle, however I can offer additional help if you choose to attempt it.