I thought iawn was used to connote "very good" or "very fine" and "eithaf da" for ok or fine.
iawn on its own is fine for 'fine' here. When added to another adjective it acts as an intensifier, 'very':
- Da iawn, diolch - Very good, thanks
- Mae'r car yn dda iawn - The car is very good
- Mae'r car yn ddrud iawn. - The car is very expensive.
eithaf da means 'quite good', or 'pretty good' and is used much as in English.
Braf, diolch has just been deemed incorrect - is that because braf is only ever used about the weather? And perhaps to describe a few other things, but not people?
braf is not often used about people. If it is, it has a meaning of being big and strong, strapping, etc, more of a physical description.
They aren't. The problem here is that in Duo's font a lower case L (l) looks similar to an upper-case i (I). If you look very carefully there is a very small curve at the bottom of l (L) but not on I (i).