But he wouldn't would he?  I mean the Eye is merely a tool, a weapon of pure hate.  I doubt that Namshiel would play second fiddle to the eye, but Marcone might, which would make him less useful perhaps?

The Eye has to be fueled by hate, or love. Marcone isn't entirely emotionless, but I doubt he has the juice to do much with it. I expect he's having the same problem with magic: his control is phenomenal, but he doesn't have much power beyond what Namshiel can channel to him.

Plus he deliberately wanted everyone to know Harry had the Eye, knowing they would trust him less as a consequence and that Harry wouldn't actually use it. Getting that turned back upon him by Harry had to come as a shock and almost certainly dealt Marcone more trouble than losing the castle did.

An interesting thing to consider: Harry had to coerce Marcone into taking in a few hundred people as refugees. Those people included children. Would a younger Marcone have hesitated to protect children in the same circumstances? Has Namshiel already started to influence him?

I also consider it unclear whether Namshiel got Nemesis'ed, which would make matters even more complicated later.