Thermal pellet bypass replacement plug spring
 

Question: What will/could happen if while replacing the OEM thermal pellet bypass with a replacement plug and the spring 11-D17 (see attached picture) was not reinstalled during engine reassembly?

Do you mean you did not install the spring itself while doing the mod? I would probably think the actual pellet would have some play room(back-forth). I wouldn't leave it like that, push the clutch in(the whole time), unscrew the front bolt and install.

Low oil flow to the rotors.

LoL what a Chody move dude... I'm actually about to Re-Install mine in the STARX8 and I notice the RX8 does NOT have a copper washer and I did not even see RTV (altho it must have gotten burned off while mapp gas torching the bolt to loosen up.) So I am going to install it with some Grey RTV and light coat of RED Lok Tite and torque it with my air compressor.. All while engine is mounted and in bay.
That must be how they put it in from factory right, with some RTV and lok tite and that's it, no copper crush washer like older 13B's apparently.. (Could not find that washer anywhere)
I did not find any read on this myself and this is the closest post similar to my situation..

Mazda stopped using the copper washer (9956-22-800) from 1994 FD RX-7 on-wards...:)

My fc has one. Rx8 doesnt.

U dont really need it, a bit of rtv will do and u must use locitie blue or red at the threads. Unlike the flywheel nut which will just get tighter and tighter, without locitie that bolt will loose and when it does it will take the whole engine with it.

The Crush Washers helped with the torque'ing of the bolt and since it does spin in an un loosening way, I dunno why they stopped using it.. I put a slight bit more than just a tiny bit of RTV lol so it must have filled in as a washer or two nicely in there along with filling in the alignment slot, doubt it passed thru the threads and onto the E shaft itself, that would suck..