I assume heno is related to nos? Is he- a prefix meaning to- or this?
Correct. It's from Brythonic *se-noχt- (this night): http://www.geiriadur.ac.uk/gpc/gpc.html?heno
If it's not, it's how I memorize heddiw and heno anyway.
I think so. Effectively, at least. A Welsh-English dictionary website gives them both as being derived from an early British language.
Tonight is surely correct too. It's used in this unit!