Defoe was my first comparison for him too.The way he moves and is able to strike the ball with either foot, and is a bit of little terrier. He most reminds me of Jermaine Defoe.
A natural predator in and around the box, dangerous from range too, with a variety of finishes.
I think we're in a difficult situation with him. I don't think it would do him any good sitting on our bench and getting the odd few minutes here and there. But if we believe in him, it means loaning him out for a full season and then persisting with Origi as back up for another year.
I can't see him playing was well in front 3, unless you're dropping Mo and Bobby. It's similar to how Studge (another 9) never played that well with Mo (especially) and Bobby in the team.
Mane is a more flexible player and can play well with any striker.
I agree also with your point that he won't work very well with Salah, but I'd bring him back as first choice backup over Origi. Origi has hit a development wall, and will not proceed further. Time to sell.
Brewster is different from all 3 of our first choice attackers, but I think that might be a good thing. We need to add more variation to our play. Having a natural fox in the box can only add to that.
I have some concerns about him fitting into a three man attack as most players of his type typically thrive in 2 man attacks. I also agree with you that he'll probably not fit brilliantly with Salah because Salah is usually making a typical striker's runs, as opposed to a winger's.
That said, I think if you added another Mane like winger (say Ismaila Sarr) and added another technically brilliant midfield player (Thiago), you could offer significant variations to our attacking play. A three man attack of Mane-Brewster-Sarr is feasible, which uses the central midfield of Thiago-DM-Keita do the playmaking (as opposed to the full backs and Firmino).