I thought it was a post-Vader Anakin. His lesson to Ahsoka, in my eyes, was that his life, her life, as soldiers led them to a "fight or die" mentality, though maybe "kill or die" is a better way to put it. That's what drove him to the dark side. Only by the end of his life, as he learns from Luke, there is another option.

"You're never too old to learn" isn't just in reference to Ahsoka's age, but in reference to this last lesson he learns from his son, that he now passes onto her.

Because that sure didn’t seem like a ‘redeemed’ Anakin.

I thought it did. The world between worlds is tied to the cosmic force, so "force ghost" anakin could probably appear however he wanted, so sith eyes and red saber were used because thats what Ahsoka was aftaid of, and confronting fears is very Jedi - Yoda went through a similar experience fighting a manifestation of the dark side within him. Additionally, Ahsoka's fear was also impacting what we saw on screen - I don't think he was choosing to flash into Vader, I think that was Ahsoka projecting.

The World Between Worlds