ok h.sie now I get it

the whole NSS_Uboat2D.val is wrong because it was copied from 2A/B file
the way You are now trying to solve this makes sense
I'm completely new to this Silent 3ditor too, but in case you don't have time to do it from the beginning...
here is what I did - I changed ALL Id's and Parent Id's in NSS_Uboat2D.val, so now it's exactly the same as original waterstream mod + now it has exhaust (the idea is the same as yours I just did this another way)
this file should finally do the trick:
http://www.mediafire.com/file/zenm1r...SS_Uboat2D.val
one more thing that came to my mind.. Racerboy released single exhausts for type II subs, I'm going to use this as an option for W+E Combi mod for my personal needs, but if you are interested in adding this as an option to Combi mod:
http://www.mediafire.com/file/ejjn2gyzmj0/W+E Combi Single Exhaust for TypeII.7z
I didn't even have time to try if it works