I wonder, if Noah knew about Clarisse's eventual death sentence, would he still insist she keeps her powers secret? At the end of the day, aside from getting adopted by the duke, having a useful, unique power would be one of the few reasons to pardon her. Keeping her alive is troublesome because she's a legit royal from Grezekaia, so any insurgents and freedom fighters could use her as a figurehead to try rally people and restore their country. So, any reason to keep her alive would need to outweigh her political burden.
damnit, just when I started to think "looks like Noah has at least one ally in the mage tower" the fucker calls him creepy punk...
I'd say it's obvious he was no ally. He's Noah's late master's son, so if Noah showed a lot of talent despite his young age, and gained much of the old man's attention, the real son would easily become incredibly envious. He might also be worried Noah would become the next master of the mages' guild. On the other hand, if Alistair was an ally, despite everything, why would Noah feel so isolated and hated to the point of wanting to kill himself? It could be that Alistair was keeping an amiable public face, for his own reputation's sake, but privately made it clear enough Noah is alone.