They are not, they are just supplying engines. If full P1 program materializes from Toyota then this engine deal with Rebellion & possibly others will be completely separate from that (according to Mulsanne Mike).
I know, and technically the VW Käfer with its flat-4 was a Porsche too. But atm the flat-4 is Subarus trademark engine.
I once daydreamed about a Subaru WRC engined LMP2, low CoG, longliveability, sound
Yes and no, but LMP2 engines must be modified production engines, while LMP1 engines can be designed specificaly for the race car.
Take for example an F1 engine. How many engines exist with more displacement than the F1 engine? And the F1 engine is still more powerfull.