Heyman as a straight up face manager, or "Advocate", would not work. Heyman is the best none wrestler heel on the mic in WWE, and in my opinion, ever. He can go into a town like Philly where he is beloved as the mastermind of ECW, and within 45 seconds of his promo have the crowd booing him, he is an evil genius! He could easily be more face like and get the crowd behind him when he talks, and do it better than many babyfaces in WWE right now. But, at the end of the day you wouldn't trust it, it would feel unnatural, you would be waiting for the manipulative, snake-like Heyman to turn and make the whole WWE universe look like fools.
Now as for Lesnar, as a face I think it can totally work. As long as WWE does one thing, leaves him alone! Lesnar as a face should be almost the same as heel Lesnar. Show up, talk little, and kick a lot of ass. The only difference, he destroys more heels than faces. Lesnar as face, is a perfect example of what I hoped Roman Reigns would become, yet WWE get a hold of him and tried to turn him into the John Cena kind of Babyface.
As for Roman Reigns part int he equation, yea he could work as a heel, but I find that to predictable. There is a long lists of former young babyfaces in WWE who struggled out of the gate, and turned. For some it worked out great in the long run, The Rock, and for some it nearly killed their career, and all I'll say for the latter is...Feed Me More.