Why was Ethan Glover not killed by Aerys II like everyone else in Brandon’s party? Clearly he survived long enough to ride with Ned to the Tower of Joy.

I don’t know. My guess is that it’s a bit like how Tom Paine avoided being guillotined during the Terror – a chance bit of luck that kept him off the list or out of sight while the executions were going on. 

In Ethan Glover’s case, I think Aerys had a very limited attention span and would drop in and out of manic phases, so Ethan may have been lucky enough that by the time it came to him (after all, Ethan as the son of a master, is below the others of Brandon’s companions in rank) Aerys had lost interest and thus the order wasn’t given to have him killed.

Instead, he was just left to rot in the dungeons until Ned took the Red Keep and freed him.