Shouldn't there be a soft mutation after rhaid iddi hi? i.e. wyno?
There is, "Cwyno" is the base form of the verb. I'd recommend using "Ap geiriaduron" on IOS and Android which can tell you if a word has been mutated when you put it in.
I was wondering where the mutation was and then realised it must be cwyno!
Would "Rhaid iddi hi gonan", be acceptable here? I picked up 'conan' from watching Pobol y Cwm.