If the redcare has its ADSL filters fitted then A/B or the smaller faceplate will work
Alarms are often meant to be hardwired so that it is a bit harder to remove from the line. Not sure what the alarm companies dictate currently. More of a policy than a technical issue
It's OK, found the best practice information here: http://www.redcare.bt.com/Resources/Installation_sup...
I also think it should be best practice for the alarm installer to attach a warning sticker to the master socket saying that removal of the lower half of the plate will activate the alarm and to call them first.
Too often ISPs respond to broadband issues by asking the subscriber to remove the lower half of the plate and insert a dangly ADSL filter to "cure" the fault. All very well, but in doing so you may activate any alarm fitted and secondly the MK2 faceplate needs to be also removed to get at the test socket.