Yes I still remember watching Liverpool’s pre-season training from a couple of years ago and someone misplaced a pass during a drill. Milner went ballistic (might have been at Tsimikas) - not in a mean way but in a clear sign of “You’ve got to be better than that”.
It was a tiny little moment, perhaps one that many people who even watched the video wouldn’t care about but to me it showed the value of Milner. Even more than being a good player it is his job to maintain standards. Push them higher, demand the best. Of course that is also Klopp and his coaching staff’s job but we have all worked for bosses who demand a lot from us and it doesn’t quite hit the same as having a co-worker - a peer - demand more from you.
I think that’s the value of bringing these older players into the reserve teams. If Spearing is coaching and playing then he will be in those sessions, he’ll be doing the same things as the players and it will be his job to make sure he is setting the tone, working harder than everyone else and demanding that they are better than him - cue easy joke about “if they aren’t better than Spearing then…” - but I’m talking about attitude. If someone is sloppy it’ll be his job to check them, if someone is lazy it’ll be his job to give them a rocket, if someone is struggling it’ll be his job to encourage or motivate them.
As you point out Spanish sides do this often. That great Barcelona team came through a B side with lots of experienced pros nowhere near good enough for Barcelona but what they did have was a good attitude and set a standard every single day.
As for the technicalities yes you can play 3 overage outfield players + one overage goalkeeper in the U23s league.
I remember the Milly/Tsimi incident as coming in a pre season friendly. Milly made a run and instead of playing him in Tsimi played the pass elsewher. Prompting Milly to shout something like “fucking hell Kostas”.
Milly also slapped him in his first league start against Norwich(?). After he was caught in possession and they nearly scored from it.
Don’t underestimate the ability of people to question every single footballing decision the club makes, despite 6+ years of an overwhelmingly large amount of good footballing decisions.
Also note that he is a player with scores close to zero in the physical categories. For him to have got anywhere near the premier league level with those attributes, let alone get 50 odd games is a testament to the way he plays the game in his head. These are typically the sort of players whose experience lends itself to doing well in coaching.