Looking at
Morrigan's infinite, there are several things I noticed. One, the height at which Shadow Blade props characters seems perfect to continue juggling the opposing character in an infinite. If Shadow Blade props characters lower, it's less likely they'll be at a height where another Shadow Blade would connect.
Another is that it seriously looks like Morrigan is canceling any sort of ground recovery into Shadow Blade. I think if we removed that ability, it would remove the infinite and allow the opposing character to hit the ground properly.
And lastly, another fix could be to allow Shell Kick to force characters to fall slightly faster. Like, instead of juggling at more or less the same height for each hit of Shell Kick, each hit of Shell Kick causes characters to descend more with each hit. That way, they'll be too low on the final hit for the Shadow Blade to connect.
For Spencer's infinite, I'm not totally sure how to fix it without fucking up a lot of his hitconfirms/combos. Have to look into it more.