Hi Anton, thanks, it is an interesting hypothesis. Just a minor note: while it possibly explains the furin site, it does not explain the pangolin RBM.

So the HKU9–1 isolate that has the CCTCGGCGGG motif was extracted in 2005-6 from Rousettus lechenaulti bats in Guandong. It is a 2d coronavirus, while CoV2 is 2b. Their spike genes are quite different:

With such differences, I am not sure how likely HKU9 and CoV2 are to end up in the same cell, but it certainly cannot be ruled out and warrants further investigation.

Thanks again for pointing it out!