11 SCHs all using MV at the same time would knock it down below the time needed to reach even the first tick after initial damage. This also makes me believe that rather than decreasing the time a helix lasts, MV reduces the number of remaining ticks by half. Seeing as how 11 SCHs still got a tick off afterwards, my theory is that it will never reduce it beyond 1 tick remaining.
This is not how it works; you can go test it yourself easily. Helices have a random duration between 45~90s; it's not clear exactly how it works, since the random durations between the high/low values do not conform to the standard resist reductions of other status effects.
MV does exactly what the description says; it doubles the damage dealt by the Helix on each subsequent tick while reducing the time remaining by half. It does not affect the number or frequency of damage ticks; if your Helix ticks with 15s left til it wears and you use MV, the effect will wear off before it deals further damage. This is why stacking 9 MV appears to be the target number; it multiplies the initial damage by 512, enough to kill AV, while still allowing a long enough window to land all 9 /JAs. Any more and the window in which to land MV becomes too small, resulting in a wasted attempt of 0 damage. As such, this is why MV merits are so important; with these, each MV usage will reduce the time remaining by 25% instead of 50%. Without these, the duration will become too small and the Helix will wear off without dealing any damage.
According to others that have researched this strategy, Helices appear to tick on 10 second intervals from the instant the target appears on the server; If you land a Helix on a target 5 seconds after it pops, the first tick will occur after 5 seconds, then every 10 seconds after. Knowing when this tick occurs is extremely important, as 9 MV will reduce the duration to the point where you have to coordinate all 9 to land just before the next tick deals damage. This is why the coordination needed for a triggered mob like AV is so difficult; the exact time that the helix will tick is different every time it pops.
I suggest reading up on the BG thread concerning this if you're interested, it goes into further detail on some points that Kaeko didn't have the time to expound upon.