Wedyn is from wedi hyn (after this).
You'll also hear wedi 'ny in the south, from wedi hynny (after that).
Could be, as long as you mean the adverb not the preposition e.g. Es i i siopa yn y dre ond dod adre wedyn (I went shopping in town but came home after(wards)/then).
I've been pondering wedyn. According to Geiriadur Prifysgol Cymru, it can mean afterwards, after this, subsequently, later, then; also, on the other hand, after all. (http://welsh-dictionary.ac.uk/gpc/gpc.html, search for wedyn)
I'm wondering if our English phrase, Well, then... is taken from the Welsh, Wedyn? I can easily imagine an English speaker mis-hearing Wedyn, taking it to be Well, then. Otherwise, I cannot think of why we would preface then with well. Why not just say, Then?
A cursory search for the phrase "well then" has yielded nothing so far.
But, at least it provides for me an easy memory hook: Wedyn sounds like well then, so I remember that it may mean, then.
It's a nice theory, but I very much doubt it. Both well and then are native English words (cf. Proto-Germanic *wala and *þan) and their usage makes sense in English and isn't at all similar to how wedyn is used in Welsh. It's just a coincidence that some of the sounds in both phrases are similar. If it helps you remember the Welsh word though, that's not a bad thing.