Hello again

Martin said :
"As a young man, I grew impatient with Mages Guild restrictions, as did many of my fellow apprentices. We threw ourselves into the riddles of daedric magic. We hungered for forbidden secrets. Knowledge and power were our gods.You can guess the rest. We got in over our heads. People died. My friends died. I've put those days behind me. But the bitter wisdom that one has been a fool is not without value. "

Can we conclude that he once was an apprentice of the Mages Guild, and was kicked off after his mistakes lead to people's death ?

In Daggerfall, high rank members (not apprentice) could invoke Daedric prince in Mage Guilds Hall. Is there a lore reason for it not to be possible anymore ? Could there be a relationship with Martin's fault ?

Thank you
Cogite

They left the Mages' Guild themselves to become daedra worshipers.

Check Martin's class in the CS, if you use the PC. His class is Conjurer, like the blue-robed NPCs found in caves. He was once one of these people. He left the Mages Guild to gain more power by trafficking with Daedra, but saw the error of his ways.