(Disclaimer: I'm in my 20's and obviously do not have the life experience you and many others in this thread do. I'm merely adding context for what your son may be feeling as someone who grew up with technology privacy top of mind as a parenting debate.)
Speaking as one of the first children to have grown up in the smartphone era, I think you're asking for a bit much. Granted, 11 is quite young, and at that age my parents did have my username and passwords to things like social media accounts.
But they did not look at my text/IM messages, and even they quickly felt like logging into my social media accounts was an icky-feeling invasion of privacy. Our compromise was they would make an account themselves and follow me so they could see everything I posted that was public/easily accessible. Eventually, they realized I was trustworthy and don't check in at all now.
I believe the YouTube situation falls under this, and I think it's reasonable that you'd like to at least follow along to what your son is posting publicly. However, as others have mentioned, some disagree. I think this is a 50/50 ball worth clearing up. However, know that even intelligent minds disagree on this. I have a lot of friends and colleagues who are adamant that a right to pseudonymity is this era's "right to privacy" and that if someone wants to have a non-associated public-facing persona, they have that right.
Besides that, though, I'm going to be honest and say that I feel very strongly like you have no right to see your son's messages and peruse through his cell phone without reasonable cause for suspicion. (For example, if you hear him chatting on the phone about illicit activity or messages, I think it's entirely reasonable to demand he unlock and give you his phone. That may be something worth making explicit since it's not looking like you're going to get everything you'd like.) However, please keep in mind that he and his phone are not games to be won. Your rhetoric, even written, goes quickly from "I want to see what he publicly posts to make sure he is not doing anything stupid and harmful" to "my ex wife may be sending him information to be used against me, so I need explicit permission to be able to take his phone and look at things at any time." Those two things are not even in the same ballpark, and the first point segueing into the second makes you seem like a less rational actor.
The best analogy I can come up with is this: What if your father wanted carte-blanche ability to come into your room at any time, without warning or notice, and look through every drawer, closet, nook and cranny "just in case" your mom or siblings or friends had given you something dangerous?
Sure, it's probably an unspoken understanding (it certainly was with my parents, who had no problem revoking access to phones or tech, grounding me, looking through my things, or otherwise being more stubborn than I when they felt like I was withholding information or being difficult in a situation they were justifiably concerned), but I hope you see how it can seem unreasonable and frankly a off-putting and intimidating when it is emphasized with an "I can do this at any time, remember that" on a consistent basis.
There were certainly times when my parents were justified in their concern and cautiousness, but there were also times where'd I'd get home from school to see all my things rummaged through with no provided justification (or a thin one, like "so and so's mom was talking about kids drinking at the party you were not at. I wanted to make sure you had no alcohol in your room."). That only made me communicate with my parents less and feel very angry. Frankly, it wasn't good for any part of our relationship. (For reference, I did not drink in high school. I spoke with my parents fairly opening, and I made that very clear to them. To know they did not trust my word to the point of blatantly going through my possessions was incredibly hurtful and made me less willing to share things, since I thought they wouldn't take my word for things anyway.)
Honestly, my first impression from reading everything you say is that you are more paranoid about your ex-wife and your son will be collateral damage in all of this. I've obviously never been through a divorce, so I can't speak to how well-founded or not your assumptions are, but I'd really consider the message you are sending to your son with the tone, rhetoric, and tenacity of this argument. He has feelings and autonomy too; don't make him feel like a piece in a game you'd like to "win" more than your son whom you love.