It certainly a contradiction to have nú go bhfuil and nú tá side by side in the same context. I was thinking nú go bhfuil would be right in both parts of sentence, standing for "nú (más amhlaidh) go bhfuil". I hope Labhrás comes on this forum; he hasn't been here for some days.

I don't like "nú go bhfuil do mhadra tar éis bás a fháil", which is wordy.

Nú go bhfuil do mhadra t'réis bháis - would seem simpler.

I think nú má tá would be unquestionably right as a variant. I think the nú tá of the original was just wrong, as tá cannot link up with the previous mura bhfuil properly.

Does this work? úinéir an mhadra so is definite, I’d rather expect mura tu úinéir an mhadra so, with the pronoun before the definite here (but to be fair, I’m not entirely confident in my understanding of the copula with 1st and 2nd person pronouns…).

You are right - so you should be entirely confident of your understanding of that. Thanks for the correction.