That's one way of putting it!
With llyfr/lyfr, I get that, underlyingly, it's one syllable, but realised as two, with a rather helpful epenthetic vowel easing our pronunciation woes. That would explain 'l(l)iver' rather than l(l)over' - though I have heard it pronounced as the latter, suggesting that some speakers have re-analysed it as being underlyingly di-syllabic.
With 'gwlith', I've never actually heard it pronounced, so can't judge for myself whether native speakers pronounce it as two syllables but 'feel' there's only one, or if it's genuinely monosyllabic (using the labialised 'g', with no intervening vowel before the 'l').
Sorry, I realise I'm getting into phonology-nerd territory now but, hey, it's certainly far more interesting (to me) than what I'm supposed to be doing at work... And, if nothing else, this new word is now very much committed to my memory!
As you were, folks!