Diolch yn fawr iawn. That mix and match question was really the one that drew me into this discussion and then I started to read a bit too much, without the background in linquistics to understand all the terminology. I got the impression that historically these were quite different and have now converged, because one of the usages has declined in use.
I also started to find examples which on GPC were referenced as "Ar lafar, e.e. yn nwyrain Morg. ac yn Arfon", which are not the usual dialects that you see together, which I found a bit intriguing and to me suggested that one wasn't derived from the other, or by divergence from a past form, but that both baswn and byddwn were probably in use everywhere, side by side once upon a time.
Anyway, I now have an extra reason to stick with byddwn, it seems a safer bet if it has to be used for the habitual past as well.