This looks good, could my comments be addressed ?

It feels ok to me, yes.
Maybe there is a need to add some autotools env variables too, or maybe those should go in Templates/config.site.

Using must is a no.
This rule should not be absolute.
You can advise people to not put a variable, but variables may need to happen from time to time.
The WWW variable is there so make(1) can create things out of it.
The ports tree is to be used by developpers, not end users. End users have packages and should not have to interact with the ports tree ever.
If a developper needs to know the actual value, they can run make -V WWW, we could even add a few targets, like make open-www that would do stuff like xdg-open "${WWW}".