Is adhering EPDM foil directly onto a slightly porous layer of bitumen to fix a leaking flat roof a good idea?
Your new roof needs to “move” independently from the old roof. In order for your new single ply membrane to be separated from the old roof, you need to add a “slip sheet”. 

I’d use a geotextile fabric and fasten it down “mechanically”. That is to say, use a special fastener to screw it down. It’s about 45 mil thick and the single ply membrane can be attached to it. 

The slip sheet can be extended up a parapet wall, around vents, over curbs, etc., so the new single ply roofing can completely seal the roof. 

How the single ply membrane is attached to the slip sheet, depends on the type of single ply roofing you use. 

BTW, you can take this opportunity to add rigid insulation down before you install the single ply membrane too. However, make sure vapor barriers, venting (or eliminating it), etc. is proper before choosing this installation.