Would “Dw i 'di bod yn dysgu Cymraeg am DDWY FLYNEDD” mean that I’ve started learning Welsh two years ago and I am still studying it?
Yes - “I have been learning Welsh for two years”, which does imply that you’re still learning it.
And does “Dw i ‘di dysgu Cymraeg am flwyddyn a hanner trwy SSiW” mean that I’ve studied Welsh for a year and a half, but might as well be three years ago?
And does this automatically mean I’m still studying it?
Yes - “I have learnt Welsh for a year and a half through SSiW” - but there is no other time implied in neither the how-long-ago sense nor the still-studying-it sense
Or if it’s something in the past, should say, for example:
“O’n i’n dysgu Saesneg am DDENG mlynedd yn yr ysgol?”
Yes, this is fine, although there are other options for the same thing - your sentence is “I was learning English for ten years in school” but you could say “O’n i’n dysgu Saesneg yn yr ysgol am ddeng mlynedd” (I was learning English in school for ten years - the word order sounds better to me), or perhaps better still, because school is ‘completed past’, you could say “nes i ddysgu Saesneg yn yr ysgol am ddeng mlynedd” (I learnt English in school for ten years).
p.s. BTW I’m also testing if I remembering mutations and changes in year/years right!
see the bits I’ve capitalised above (don’t worry too much about the ddeg/ddeng one - it’s a bit of an odd one as far as mutations go!)