Very short answer is;

SNR can swing over many dB's and its normal though could be an indication of an external problem (interference) or a line/hardware issue. Generally (in my part of the UK at least) we drop 1 or 2 dB of a night and maybe more in the winter time.

Generally if you swing from say 6dB margin to 3dB over a matter of moments then you might be seeing some sort of external interference.

It may also be that the exchange hardware is doing something with its power settings when you restart - see if you can see the power output and watch what that does.

As for the modem reporting a different attenuation that may be to do with your router firmware. I have a draytek that will report a different line attenuation depending which version of modem code I'm running.

Actually the modem will report signal attenuation at 300kHz and not line attenuation. They may be similar but not the same.