FYI, there are actually more than 2 bottlenecks in the crossing species of a zoonotic virus. Spike protein and furin cleavage site are not the only limiting and/or permissive molecular factors.  In fact, any changes in any particular nucleotides within the Spike and furin site will affect the effectiveness of the virus entry into human cells, effectiveness of the infectivity of the zoonotic virus!  Full scientific investigation will determine the full spectrum of the sites effectiveness!