Justin wouldn't wonder why the Wardens were nervious about Harry... he would chock it up to two things:
1: Harry's well above average (for the council) raw power level.
2: Justin faking his own death at the hands of Harry.
Justin above anyone else would 'know' Harry was strong, but not as strong as the Wardens make him out to be. The comment wouldn't even occur to Justin as cover.
I will eat my hat if Cowl IS Justin... no scratch that, I will go buy a hat and then eat it...
Justin is almost as bad a candidate for Cowl as Nameless is.
But Harry's raw power when Justin last fought him would have been a lot weaker. Not to mention his skill level would have been a lot worse. Harry finished his education with Ebenezar and then had over a decade of honing his skills and growing in strength until he would meet Justin again (assuming Cowl is Justin). So, not unreasonable then for Justin to wonder about Harry's power level. He would have heard Harry was strong, and he would have known his potential, but he wouldn't have yet seen it in action. Like a college scout seeing a pro footballer in high school. You can see what they might be, but you still want to see the finished product otherwise it's just unrealised potential.
Not sure about your point number two. Are you agreeing that Justin isn't dead?
Why else is Justin such a bad candidate? He was considered a powerful warden, we know he experimented with dark magic, he was at Kemmler's final fight (which leads to all sorts of questions if he is Cowl), he is a capable swordsman (being a warden) and Cowl speaks of his skill at swordplay, Justin picked up Bob (a demon that served arguably the greatest and most feared dark wizard in history) from the ruins of Kemmler's last stand (and how did he even know of Bob, and why didn't he just destroy it, and considering how strong Evil Bob is how did he even control it? And how did Cowl know Harry had Bob...Justin would know this of course but who else could know?) - there are so many reasons why Justin makes a good candidate. And those are just in-universe ones. From a writing perspective it's actually a classic literary trope to have the old mentor as the villain. Consider how much more impactful Cowl's identity reveal will be on Harry if Cowl was revealed as Justin. It would anger and scare Harry, it would tear him up emotionally that his surrogate father-teacher is still alive but not only alive but actually one of his biggest enemies. Not to mention Cowl clearly is working towards some skewed version of a Greater Good. He is insane but not like Corpsetaker or Grevane. He wants to improve the world (apparently through stopping death for some reason). And Justin might just have a very good reason for stopping death. We also know Justin was involved with Harry's mother, Arianna and Paolo Ortega, Lord Raith etc. We know they had some scheme around the Stars and Stones...just like Cowl. Also, consider that Simon was killed in his fortress. The White Council suspected Harry of supplying the Red Court with the necessary information to get past the wards...because he was Justin's student, and Justin was once Simon's student. Justin was only ruled out because of his apparent death. Yet we know Cowl and the Red Court worked together, and we know Harry didn't give out the information. So unless Simon is Cowl, or some other student of Simon was involved (the least likely story-wise), Justin makes for a good candidate. Cowl hides his face because he doesn't wish to give out any information. Justin is supposedly dead, so fairly good reason to cover his face. Cowl wants to bring down the White Council for having too many restrictions, Justin also felt that the Council has too many restrictions (as he learned from his master too).
Kumori as Elaine also works very well, given Kumori is both powerful (like Elaine), friendly with Harry (even though she shouldn't really be), quite likely was involved in Aurora's corruption (which Elaine was in a prime position to do), Kumori picked up Harry's force rings (which Harry remarks almost no-one picks up, but Elaine would given she studied under the same master and would know Harry's style and training), and Kumori is tall enough to hold a knife to Harry's throat (which is tricky for most women given Harry's height, but Elaine is at least six foot), and if Elaine is Kumori she is far more likely to work for Justin (as Cowl) than some other Cowl (because what link would Elaine have to some other Cowl?) Kumori as Elaine is still the most popular theory, and for good reason. It's the most likely. Also adds to Harry's betrayal and isolation.
If not Justin, who do you believe it is? Who is so familiar with Harry who is a powerful and well-trained (White Council level) wizard, who also might have experience with necromancy, who hits hard (perhaps harder) than a Senior Council wizard?
I honestly had never considered Nameless. I am hoping CT will explain his idea further. It's intriguing, even if I can't yet see it myself. One of the main reasons I don't think it works is that Nameless is closer to being a demigod and but Cowl's magic feels human (even if it's incredibly strong).
It’s likely Justin was working for Cowl, we know Nameless worked with Kemmler, he went to winter but couldn’t take Bob with him without incurring Mab’s wrath, so he handed him off to Justin. Justin may have only joined the Wardens in the first place as Nameless inside man.
When did Nameless work for Kemmler? Also, assuming your theory is correct, why would Nameless use a dark wizard and later hexenwolves as bait to flush out the information that Harry has Bob? And why would that reveal Bob is in Harry's possession?
Justin got Bob from Kemmler's effects after the White Council dealt with him. If Nameless had ever possessed him, he'd have told Harry in The Law.
Agreed.