Good Points, I'm sorry to say the Clans Do use stealth armor on Battle Armor see TRO3075 Aerir PA(L) for 1 (if the only 1 I can think of).
As that does not use a ECM would a ProtoMech system ?.
As I said, I think a Proto stealth armor system would likely be based on the Clans' BA stealth armor systems, so no, I don't think it wouldn't need its own ECM unit for that purpose. There would need to be some kind of compromise, though, to keep the stealth armor balanced...internal space taken up or something.
As I typed that last line, it struck me that the Clans have/had knowledge of the SLDF's Null-Sig Systems. Perhaps they could develop Proto stealth armor based on that technology. Of course, that raises the question of why the Clans don't use stealth or Null-Sig on Mechs, but I think Karagin provided us the answer to that in another thread: Mechs are too big and loud. Protos are smaller, and therefore better able to make use of stealth. But of course, we are left with the question, WOULD the Clans do it? How would stealthy units fit into the Clan rules of battle and their sense of honor? I don't know...
Be careful what you wish for. I might let you have it.