I threw out "Maitarussa" without much thinking about it there, but how
would a Quenya form of Maedhros look, if it was derived directly from the original Primitive Quendian name?
S
Maed comes ultimately from PQ
Magiti, the same source as Q
Maite, which takes the form Maiti- in compounds.
S
Ross comes from PQ
Russā, which also produced Q
https://eldamo.org/content/words/word-1387000543.html, which is well attested at the end of names.
So, fairly simply, the Quenya name of an "ancient Maedhros" would be Maitirussa. I was nearly right! Back in Cuivienen he would have been Magitirussa, which does seem a bit overwrought for the culture that gave us "Indis and Ingwil, children of Ilwen and of Ingwe son of Ilion", but maybe the Noldor were just showoffs. Or, maybe more likely, Magitirussa was an
epesse, like Sindacollo or Therinde, and his true personal name is still unknown.
hS