What you are doing will work. They will glue the fabric onto the metal, you won't need tabs. As far as the "incline", which is called pitch in the furniture world, the simplest thing is to angle the base, not the foam, but it can be done either way. Yes, the foam will be glued to the plywood. You should drill some 1" holes through the plywood to let the air escape through the plywood, both on the backs and the seats. Either that, or cut out large rectangles and criss cross the openings with elastic webbing.
If you can't find foam to your satisfaction on the internet, I can get you whatever you want from my supplier and have it drop shipped right to you. One online source for foam is Rochford Supply. CLICK HERE
The firmest they have is 61 compression which should work O.K.