-The Jab didn't combo into footstool.
-The Bair knockback characteristics depends on a DI and a character's gravity and weight.
I'm a frank guy so I'll say: this won't take Marth anywhere.
However, after the Bair a neutral tech (and even the backward tech if at the ledge) can still Fsmashed. The forward tech can still be punished.
However, depending on the character, they may not be forced to land after the Bair! They could get out of hitstun while airborne which means they can jump out or input an aerial/special before they land.
Bair might not be optimal here. Care to share what percentages Lucina was at? Depending on that, this could be a landing Uair to tipper Fsmash or backwards falling Uair to tipper Usmash situation instead of the Bair. Or just do landing Uair to tipper Uair/Bair.
All in all, props to you! This is very creative.