As a Quins fan, I undoubtedly have some bias, but as an England fan, I want to see a better scrum. I think that's an objective fact that the Quins scrum has been very good this season which is backed up by the scrum stats.
I'm assuming there's no debate on Marler?
Sinckler and Stuart are better players than Collier, particularly when it comes to their contributions in the loose. However, I don't think it's a stretch to say Collier is a significantly better scrummaging prop and is currently in excellent form which can't really be said for Sinckler or Stuart. If Eddie wants a nuts and bolts tighthead, Collier is currently putting his hand up for selection.
As for Walker, I'd argue the 3rd hooking berth is very much up for grabs. Blamire has barely played since the AIs, Dolly is playing a bit part role behind Montoya and Langdon is nowhere to be seen ATM (I assume he's injured). Singleton is starting every week for Gloucester but is a bit hit and miss. Walker is playing well every week and, importantly, our lineout is getting better (100% last night). He's not a shoo-in by any means, but he should certainly be in the conversation.