Can I connect a second receiver to the control panel? (e.g. In the case of bad reception)
Yes. You can connect several receiver boards, for the control panel it is the same if it has 1 or 2 or 5 receiver on the buss.
There will not be a multiple alarm from several receiver because there is a short lockout for the message from the receiver and the panel will only relate to the first one.
If I (the installer) replaces the receiver board is there a need to learn the detectors again?
No , when you learn the detectors the radio code is saved in the control panel memory , and not in the receiver.