So "bhean" means woman AND wife?
Yes, although colloquially just 'wife' :)
Why does "bean" change to "bhean" just because " a' " is added . . .
a' makes it a definitive article (= "the"), so I guess that's the rule:
Is that the rule for every noun (starting with b, c, d, f, g, m, p, s), though?
The tips for this topic give a good explanation.