Should this be 'deg orennau', or do we use the singular when counting?
Yes, up to ten, singular is used, it seems to include ten. But the construction "deg o ..." (ten of ...) would need plural!
Why is it dw i eisiau instead of dw i'n eisiau? Is there a rule or is it irregular?
Is this what you might call polite in welsh, in the sense that in English in a shop you might say not "I want..." but "I would like" or "je voudrais..." not "Je veux" "Ich moechte/Ich haette gern" not "Ich will" etc?
quite interestingly, "isio" is used here, instead of "eisiau"! I'll try this out in other places.