Apparently Mako is in France on holiday - not sure if he's specifically taken himself out of contention for a callup or if it's just the logistics of it, but from what I understand he's not been considered for a callup at any point
I rate Bev very highly but realistically he's a promising youngster, not an international starter - he's got surprisingly good technique for someone so inexperienced but doesn't have the size/strength (yet?) to go up against guys with a decade more experience than him. I remember vs La Rochelle last season when the combo of Atonio and Skelton just pushed him straight backwards with basically no resistance - literally men against boys
I'd honestly start Dolly, he's easily the best setpiece option - though I think, as you said, Singleton will possibly(/probably) go straight in to start as the more experienced player
I think the composure thing is a bit over egged - definitely a hot head in his youth but doesn't flip out more than any other forward these days IMO
He's definitely not known for his scrummaging, though, as you said - he'll do a job against most sides but up against a real set piece specialist he'll come off second best