I'm too lazy at this time of day to go poking around through pages to find quotes of things that have been brought up multiple times. Sooo...
Alex should be strong enough to easily overpower Holloway, except for the fact he was hit in the head, hard enough to knock him about for an unknown amount of time. Aside from being extremely painful, something like that could possibly cause decreased motor skills, dizziness, nausea, etc. Not to mention the sheer shock of someone you grew up around, someone you trusted, someone who was nice to you when few others were suddenly go batshit crazy and try to kill you. His physical capacity is going to be diminished.
I play on xbox360, and Elle won't go in that room for me either. However, it's obvious that she knows at least some of what her mother is up to. After all, Elle is suffering the consequences of failing to convert when Alex finds her, and the first words out of her mouth are "my mother!" to which Alex replies "Elle, I'm sorry." She probably has some idea that her mother's dead, and she'd be suffering from the same aforementioned shock as Alex.
As for Alex not limping, it doesn't bother me. Characters get hurt in games all the time and pop right back up like nothing happened, this time it just happened during a quick-time event rather than regular gameplay. Besides, his focus shifts to Elle. People suffering from extremely painful injuries have been known not to feel the pain in high adrenaline situations- of course its an exaggerated effect, but it's a game.
It's not my favorite part of the game, but I think it works. She displays a whole new level of hatred and insanity. Bitch needed to die, and it wouldn't have really worked to have her go down in a typical battle. It also doesn't come off as a calculated attack. No matter how awful she is, there's a certain foulness to killing a maternal figure. The moment's desperation excuses it somewhat. Furthermore, it's not supposed be a scary moment, but rather one of shock and disgust- which it is.