Hi John
I know it makes little sense why there is foam on the back of the shutter, but over the years repairing cameras I've seen many things that haven't made sense...
I've only repaired a couple of these cameras, and I don't think I ever needed to replace the foam. It could be my memory playing tricks on me, as I'm not 100% sure if this model had foam on the back of the shutter.
The reason there was foam on the back of some shutters is that the space between the solid plastic light shield in film chamber and the rear lens element was quite large, and the foam was put in to prevent any possible light leaks - overkill maybe, but knowing Canon they did it for a reason sometime during the cameras production....Canon would issue a SMR (Service Manual Report) which would update any modifications/part number changes etc for a given model..
What could also have happened is if your foam on the back door was crumbling it could have fallen between the rear lens element and the inside of the film chamber..
I would suggest running a film through the camera, and if you are happy with the camera get it serviced by an old technician who has done them before...the way you have "cleaned" the residue form the shutter unit is only a stop-gap.
To clean the shutter correctly you need to dismantle the shutter and clean all the shutter surfaces of sticky residue..
Hope this helps
Cheers