Honestly the best case I've seen for Pilot & Series Alastor being 2 different characters are the responses to Angel talking about doing sexual stuff with him.
Angel: I can suck ya dick!
Angel: If you film me going at it with Mr. Fancy Talk Creepy Voice here, you'd be rolling in participants willing to stay at this tacky hotel.
Alastor: Ah-hA! Never going to happen.
First off, the talking difference. Pilot Al keeps himself to few words and he keeps up his amused tone. We don't know why he says no. Series Al forces a laugh, states the full meaning, and has an irritated tone. We can easily tell that the idea of him being sexual with Angel irks him.
Secondly, and the bit I harp on about the most,
SERIES ALASTOR DOESN'T USE A SMILE AS A MASK. YOU CAN READ HIM LIKE A FUCKING BIFF, CHIP & KIPPER STORY.
There is no mystery to Series Alastor. He is clearly annoyed. Not even just in this moment, in the whole "hey this ad sucks, Al" scene, his emotions are clear as day!
Charlie: Maybe the tone is a bit.... off..?
Alastor: ... but you insisted on this! (TV ad instead of radio)
The only instance I like is this, when Vaggie responds to his advert: I'm sorry, what the fuck?! (faces are before she starts to when charlie interrups)
It's crystal clear he enjoys Vaggie's reaction & that Charlie cuts it off, yes, but it's subtle about it in just having his smile reach his eyes more.
It feels like the smile became less an aspect of his character and more an aspect of his gimmick if that makes sense? Like he isn't doing it, Spindle are because that's his Thing. That could be because his reason for constantly smiling has changed. Pilot gives the impression that the smiling is Alastor's choice, Amazon (especially given the Green) gives the impression he's forced to. If this is the case it's fair but then Yeah these are 2 different characters. Plain and simple.
If Amazon's is meant to be read the same as Pilot's, it feels like the animation director/role that would be in charge of that either
doesn't have confidence in the audience to understand Alastor' character without the rest of his verbal & nonverbal language telling it to us
doesn't have confidence in themselves to convey Alastor's character without using the rest of his verbal & nonverbal language telling it to us