The flippy floppy part is an easy fix.
3ds Max’s default world space is “Z” up and UE4’s world space is “Y” up so to get the player model onto it’s feet you just need to set the world axis in FBX when exporting to “Y” up.
Adding a root bone will not correct for rotation, it’s a world space issue, but adding it is whats causing your second problem.
The rule is simple that the animations has to match the rigged model and is considered being absolute as part of the source chain. When exported as a complete package the hierarchy is recorded as both part of the mesh rig as well as the animation data set and the two “has” to match to ensure that any additional animations added will also match maintaining the source chain.
By adding the root bone you broke the chain as the animations do not contain information as to the added bone so to reestablish the chain you will either have to remove the bone to match the animations again or you will have to redo all of the animations to reestablish the chain.
The exception to this rule, which I understand UE4 does support, is the addition of a child bone that is not part of the parent child hierarchy.