The execution frames is somewhere between 10-15, just not very sure about the actual data. They use it as sort of a frame trap and to hide their following mix ups.
you can "see" Reptile still but it still hides visual queues for block patterns.
EX invisibility is actually very safe to cancel into, start up is probably slightly faster 8-11 frames I'm guessing. This allows reptile to jail you and then follow it up with another string.
It takes 12 blocked attacks to build a meter so Jip, 321 ex invisibility 321/1221 = 7/8 ticks.
You can utilize the normal one until you see opponents poking out, then flip the frame advantage on them with EX and land a string. EX is of course safer and completely hides you
Long story short, its just tricks.