Gilmour left because he demanded to go. Whether the hiring of Potter was planned in advance behind the scenes or not, there's absolutely zero chance a player on the fringes like Billy Gilmour or his agent would be kept in the loop about it. Boehly was reluctant to let him go but caved in when both the player and Tuchel made it clear they wanted it to happen. Could something have been done to prevent this scenario with better planning? Definitely yes.
As for Colwill (and to some extent Gilmour), even if Potter really wanted to work with the player(s) at Brighton who's to say he would fancy them to be a regulars at this season's Chelsea any more than Tuchel did? Chelsea and Brighton are two very different clubs with different quality players and different levels of expectations. Colwill is only a loan anyway so it's up to the player himself to perform in a way that makes Potter and the club think it's worth bringing him back into the squad for next season.
The club also reported it as a two year deal with no mention of an option.
But even if there is an option included in the deal, should he be about as shit a signing as you seem to believe he is, there's hardly any chance of the third year being activated so it's definitely not just
@duffer being 'pedantic'. It's a two year deal which can turn into a three year deal if he turns out great.