1: There should be a way to adjust the alarm setting in the alarm panel. Granted I'm running an older boat and an older setup but I was able to adjust when the "idiot light" comes on in my boat. Check the control board that runs your alarm lights/buzzer.

2: Bet your gauge or sending unit are funky. The temp usually decreases on my boat when throttle is increased due to increased flow of cooler water through the engine and it increases slightly at idle as I idle through the numerous no wake zones here. Weird that it cools more at idle than when running.