Yes, they’re legitimate - it’s the standard way of turning an adjective into a ‘-er’ ending. There are irregulars of course (as there are in English).
There’s also the standard way of getting the ‘-est’ ending - for that you use -af, so hwyraf = latest, cynharaf = earliest, hapusaf = happiest, etc. (and again, there are a few irregular exceptions)
One interesting thing is the use in place names. In English, Upper and lower (villages, farms, etc) might be the case, but in Welsh it is usually the -af ending. So uchaf (highest) also meaning the higher/upper of the two places.
Mostly fine, just a few notes! HWYRAF = LATEST (Maybe diweddarra is better fo the context of most recent? - yes, DIWEDDARAF is in context of most recent HENAF = OLDEST - a spelling variation needed here (it’s one of the irregulars), so HYNAF IFANCAF = YOUNGEST - yes, particularly in the South but in some places you may also hear IEUAF, IEUANGAF, IFENGAF, and colloquially 'FENGAF BRYAF = SHORTEST - this is an semi-irregular, so BYRRAF (the r doubling is the irregularity) CRYAF = STRONGEST - typo I’m sure… CRYFAF YMHELLAF = FURTHEST - you can also use BELLAF YNAGOSAF = CLOSEST - this is an irregular. Closest = NESAF (or sometimes but less correctly AGOSAF) DDISGLEIRIAF = BRIGHTEST - in theory yes, though I’d say most people would tend to use FWYAF DISGLAIR in preference.
As a rule of thumb, single syllable words usually take the -af (except the exceptions of course!), with words of more syllables you will often hear “fywaf xxx” instead.