But after adverbial question words such as sut, pam, lle/ble, pryd, am faint o'r gloch, roedd is used in the standard language, so I've been told.
You're right. In more formal Welsh Sut roedd is correct, because sut is an adverbial question word. However, in more colloquial Welsh Sut oedd is absolutely fine. It's like "How are you?" in formal Welsh is Sut rwyt ti?, but no one would ever say that - it's Sut wyt ti? as it's such a common colloquial phrase.
Just a heads-up: bore 'ma "this morning" isn't confined to a specific past tense. You could use it with:
present perfect: Beth wyt ti wedi wneud bore 'ma? "What have you done this morning?"
simple past: Beth wnest ti bore 'ma? "What did you do this morning?"
imperfect: Beth o't ti'n wneud bore 'ma? "What were you doing this morning?"
pluperfect: Beth o't ti wedi wneud bore 'ma? "What had you done this morning?"
It just depends on context, like English.