I'm pretty sure she is vulnerable those six frames. The grab animation cancels the shield, and there is six frames of startup.
This is correct. Once you start an action that cancels the shield (grab, jump) the shield gets canceled (who woulda thunk) and you go into your normal grab/jump animation.
I think right during the frame(s) where the grab occurs you're kinda invincible because grabs overprioritise non-grab moves, but im not sure
This is almost true. Grab does outprioritize every move. Unfortunately, shine doesn't only come out first frame, it also happens to have invincibility on that frame. So if the grab hitbox and shine hitbox "collide" what would happen is that Sheik's grab misses (because the spacie is invincible) and the shine hits sheik.
To clarify the whole frame data thing, let me just give you a few examples.
Ideal Pillar (typical pillar)
The first frame window where your shield isn't in stun is 15 frames long, from frame 11 to frame 25. Say you want to grab Falco. With frame perfect execution, it would look a bit like this:
11 start grab
12 grabbing
13 Airborne grabbing
14 grabbing
15 grabbing
16 grabbing
17 Grab comes out
18
19
20
21 Start Dair
22
23
24
25
However, you can also do it as late as this:
11
12
13 Airborne
14
15
16
17
18
19
20 Grab starts
21 Start Dair grabbing
22 grabbing
23 grabbing
24 grabbing
25 grabbing
26 Dair Hits, HitLag, Fast Fall Grab comes out and outprioritizes dair
Thus you have to be accurate to within 9 frames to succesfully shieldgrab a Falco out of a standard pillar after the shine. That's 0.15 seconds, which IMO is pretty reasonable. Nair OoS is a frame faster, but most of the time you're probably going to not do it perfectly and do it like frame 10 or something, so it's probably easier to just grab.
Early Pillar
During the early pillar, the first red window is only 6 frames long. With frame perfect execution (in other words, don't actually do this, it's just an example), you can nair OoS to interrup the pillar:
1 Shine, Hitlag
2 Hitlag
3 Hitlag
4 Hitlag
5 Hitlag
6
7
8 Start Jump
9
10
11 start jump
12 jumping
13 Airborne (start Dair) jumping
14 airborne, start nair
15 nairing
16 nair hits
17 Dair Hits, Hitlag
As you can see from this example, nair oos is pretty da
mn fast- as long as you can actually execute it well enough.
Edit:
but once falco is a certain point in the air he's too high to shield grab, right?
I don't think so. Falco's short hop is pretty low, and Sheik's grab hitbox is way too proken so I doubt this is the case.