Microwaving coffee is bad in that it changes the taste of the coffee.
However it's safe to microwave coffee and other than the change of the taste of the microwaved coffee it's okay to microwave the coffee.
I microwave and reheat coffee in the microwave several times through the day and do it everyday without any issues.
I just add a bit of water to dilute the coffee before microwaving it or it would become too strong tasting.
Day old coffee is OK to drink as long as it has no mold in it.
In most cases leaving coffee out overnight or for a day will not cause the coffee to go bad.
However the day old coffee or coffee left out overnight won't taste as good but it's still safe to drink it.
I do this everyday and every night I usually have at least a half a pot of coffee left overnight and then the next morning I add a bit of water to dilute the coffee and then reheat the coffee in the microwave.
The reheated coffee doesn't taste as good as fresh coffee does but it's still good to drink and will cause you no harm.
It usually takes a few days of the coffee being left out before it grows mold or goes bad to make you sick.
However if you want fresh coffee and the best tasting coffee it's best to make fresh coffee on the day you wan to drink it.
However for me I just want my coffee and won't waste the leftover coffee and will heat it up in the morning and drink the rest of it.
Then I make another fresh pot of coffee and have been doing this for 10 years now without any health issue or any issue at all.