That works perfectly well. “Un” can be used in a non-number referral situation just as in English. Some uses of words are common between the two languages, although it’s good to check if it doesn’t seem to “fit”
I would say not. The “have” (cael/gael) thing is too important to miss out, imo.
Although, I’d be happy to say “Does dim rhaid cael un” for “There’s no need to have one” for a generic situation…
So how about “was no need to get one”? As all the words I can think of mean get/acquire, rather than possess. Gen and gyda sound clunky ? How about cadw for keep?
Of course you could just say ‘There’s no need for one’, which would be:
Does dim angen un
or if you want to specify ‘me’, you could say
Does arna i ddim angen un
or indeed
Dw i ddim angen un
which latter may cause dizziness, vomiting and even minor strokes among certain of the language police, but is perfectly OK in the real world of natural authentic Welsh.