Is this form of the past used in everyday conversation?
Yes, but less so in casual speech.
Why is it "buoch" and not "fuoch"?
am faint o amser...? is a simple adverbial question, so it should be followed by mae or b-forms of bod, or by unmutated forms of other verbs. Some people do mutate verbs after simple adverbial questions, but that is not strictly correct.
Thanks again. That clarifies things.