No - Welsh does not make nouns plural when counting them. You might be able to say deg o orennau (ten of oranges) if you wanted to emphasize it though. See here: https://en.wikipedia.org/wiki/Welsh_numerals#Use_with_nouns
Yep, number + singular noun in Welsh. Or number + o (of) + plural.
So: dau afal, dau o afalau, deg oren, deg o orennau.
The "o" construction is usually used with larger numbers.
Can I use dw i isio instead of dw i eisiau? isio is the version I'm taught in lessons here in the North.
isio is accepted for most sentences alongside eisiau, in my experience - if it isn't, report it as "My translation should be accepted" and it will probably be added.
You will not see isio very often in sentences that Duo shows you, but you should generally be able to use it yourself.