Yep, Caecilia (the Kindle's default Serif font (used in the books, and it's also the default font in the browser)) is indeed missing some combined diatrics. (It does support some diatrics though, ie. the few ones used in french ^^).
Good news is, both Droid & DejaVu have them all (AFAICT).
So, with this font hack, and some particular sets, for example, Museo or DejaVu (And possibly other fonts, I didn't check more than that, if you want me to check a particular font before trying it out, don't hesitate to ask
), you should have a font supporting most (if not all) of the diatrics needed for romanian script.
EDIT: Scratch that, Droid Serif is missing the two 'T comma below' glyphs.
EDITē: But DejaVu does. I'll attach a DejaVu set to this post
.
If you prefer the 'Serif Slab' look of the default font to a classic Serif, the Museo Slab set in my third post should support the needed combined diatrics.