When I looked at it the back seal isn't as critical. If it were to leak a little due to shrinking, plasticizing, etc it wouldn't matter. As long as it "mostly" sealed on that oring it would be fine. I am pretty sure I remember there even being a small hole in the block where it allowed water to flow into the valley right next to the back seal. So it really isn't a full seal, wouldn't cause an external leak.
If the rear oring seal wasn't there it would however allow the pipe to move and destroy the front seal. So it indirectly matters but a perfect seal didn't seem important.

The bimmer fix should work if the seal isn't completely gone but I agree that a new seal is better than relying only on the RTV seal between the stint/front cover/pipe.

I think the bimmerfix is a good product in that based on pictures that I have seen these seals seem to fail when they get pushed out into the water pump. With the bimmerfix stint it should both prevent that (keeping the oring seal intact) and also provide the RTV seal.

So doing it as preventative is much more robust than waiting for the oring to fail because it can prevent the oring from failing by not allowing it to be pushed out from the back side.

I got great support from the bimmerfix guy. I did the fix on my engine with the uro pipe first and then when I changed the engine I needed a new undersized stint to do both fixes on. I called and he provided it right away with no charge and even shipped for free.

-Rich