The thing about twists is that they come in 3 categories which I will call the should've seen it coming*, the makes sense in retrospect** and the mindfuck*** (there's also the badly written but we'll ignore that). This whole 'MC's mother is secretly alive' theory is in the second category where it is entirely possible for that scenario to be true but, since it can only make sense after the reveal, it is something nobody can work out in advance (the second type is the worst type, imo)
*Example would be The Sixth Sense (1999) where all the clues are clearly presented, such that it is possible to work out the twist long before the reveal.
**Example is Trance (2013) where a salient fact is only revealed at the end which, while consistent with everything that's gone before, could not be worked out ahead of time.
***Example, The Usual Suspects (1995) where the use of the unreliable narrator trope means the twist is impossible to see but for entirely justifiable reasons.