As I mentioned, of 2 CMs it probably looks like:
LCM: Gana / Iroegbunam
RCM: Garner / Armstrong
ACM: Alcaraz / McNeil
Thats if Gana extends and Alcaraz signs of course.
With Rigg he's mostly played ACM, CM and then DM
https://www.transfermarkt.com/chris-rigg/leistungsdatendetails/spieler/947411
I dont think theres any harm in signing him to rotate in/out of the squad. Especially with him being a left footer he may be able to take Gana's place (by then behind Iroegbunam) in 12months time.
Whenever there is a talented English youth international available then we should be looking at it and seeing how to create a pathway.
He's 17 hugely rated and playing in most games for Sunderland -- why wouldnt we want him in the squad