Thank you for your answers guys! About the byzantines, I guess we wont have good answers as long as more archeological discoveries aren't made. There are some indirect clues that I've been trying to make sense of, but I still can't see the bigger picture.
There is some evidence that at the first half of the 14th century, the byzantines were using some kind of armor over their mails (see this discussion at reddit:
https://www.reddit.com/r/translator/...m_ibn_battuta/ ). Unfortunally, the fragment by Ibn Battuta is not clear enough to define what kind of armor would that be.
There is also the will of the Skouterios Theodore Sarantenos that implies mail (lorikion/lorikon) as the mail armor worn by a rich cavalrymen of the same period, but again the terms are not clear (he also had a kazakan/kazagand, which supposedly also had a mail layer).
The instructions of Theodore Palaiologos, which suggest for the same period a combination of mail and an unspecified cuiriee (maybe a leather armor, maybe an early cuirass, it's not clear).
Another indirect source in the painting "Saint George and the Princess", by Pisanello, which was supposedly inspired, in some details, by contemporary byzantine dress, and which depicts the saint in an archaic looking cuirass.
There is also the mention of the 300 burgundian soldiers sent to Emperor Constantine in 1445, which probably would bring modern plate armor with them.
The final indirect clue is, in my view, the fact the the Stradioti in venetian service are mentioned as having at most mail armor and a helmet, with only a few wearing a "panziere" (maybe plate belly armor).
What can we make of all these indirect clues? The more I look into them, the more confused I get.
EDIT: Apparently the only two pieces of armor in greek museums labelled as byzantine ones are mail suits. There is also the fact the the Ottomans usually kept the looted armor pieces of their conquests in the Istambul armouries or worn them in further campaigns, and from what I know, they didn't change from their mail/plated mail armors after their conquest of greece, nor are there lamellar armor pieces in the Topkapi collections (some of the captured european plate armor pieces might have been worn by the greeks during the 15th century i guess, though I don't think there are many ways to test this possibility.)