Players literally will go on strike at clubs like Southampton to play in Champions League group stage. He would have learnt more in 6 games there against top opposition than in 38 league games under Claude Puel and Mark Hughes.
He wasn't done at Celtic. It's obvious when players are too good for the level they are at, you could see it in Denayer and Van Dijk when they were at Celtic, you couldn't see it with Roberts because it was clear he still had a way to go before he'd out grown that level of football.
The idea that a player is mentally weak because they chose to play in the Champions League against Bayern Munich and PSG is absolutely hilarious.
As for your bullet-points -
- Southampton are so badly run, with such a lack of quality that they're in a race to the bottom with Stoke and Swansea.
- A club competing for nothing in the Premier League is not more high pressure than fighting in the Champions League and Europa League. No one could possibly say that with a straight face.
- The club don't care if he's of the level to play for a team aiming for 12th in the Premier League, they want to know if he can play against the best of the best. Manchester City, PSG, Bayern Munich, Juventus. These are the games that tell us how good he can be.