It's not necessary, but in colloquial Welsh pronouns are often doubled. Ei gar e or Fy nghar i, being other examples.
Actually it's the other way around. "Dy gar di" is more formal and "Dy gar" is less formal.
No. Whereas "dy gar di" and "dy gar" are both used in the colloquial language, the "di" is found much less frequently in the formal language. It'd only be used very occasionally, mostly for emphasis or to fit the metre in such things as poetry.
Hehe changed back, diolch. My bad for trying to talk so confidently about things I don't fully know about (-:
In South Wales you wouldn't it's exactly the same sound however in North Wales 'u' has a different more nasal sound, similar to French, so you could tell the difference.
It isn't more nasal in North Wales. The tongue is just slightly further back (and possibly lower) in the mouth. It isn't found in any variety of French that I'm aware of. It's the sound of Romanian î or Russian ы and similar to some varieties of Swedish i.