Why didn't she ...

a) Because she wasn't very smart.
b) Probably because she had no leverage and not much body weight to throw around. The bed was at least queen sized and the frame looked very heavy. I'm not sure she could have shimmied the bed. She should have tried, though.
c) Because she wasn't very smart.

I don't think she would have been able to get the shelf down could she? She could lift it, but I didn't think she was able to do much more than that. And yes, that bed frame looked really heavy with the posts and everything.

I want to know why she didn't try different vocal oscillations until she hit on the frequency that the molecules of the handcuffs vibrate on, thus shattering them. I hate how movies don't unfold exactly how I think they should. Jerks.

MOSTLY ... why didn't she turn on a friggin' light. Get something to eat, drink some coffee if she is going to drive. And why not just drive to a neighbor's and ask for help? Nothing made sense in this movie.

Even if she could bring the shelf down, how would it help her get the phone? She had already tried reaching the phone with her foot and she was still a distance away from the bedside table, let alone the phone itself (by the way, who has a bedside table that far away from the bed anyway 😆).
But don't forget, she needed the shelf to stay there anyway as that was the only thing that made that water accessible. If she had used the shelf then she would've forgone the water.....just to find out the phone is dead anyway.....which then also means she wouldn't have ultimately used her own blood as lubricant.

I have to agree with you about the phone charger though. I would've gone straight to the phone after but not before obviously taking in some fluids first! But I guess maybe her first thought was just to get to some help, and leave before the Moonlight Man returned.

Remember, by this point she's really disoriented, weak, and dehydrated, not to mention rather traumatised. People usually don't (or can't) think straight anymore.

I spent the whole movie trying to figure out why she didn't just stand up, slide the cuffs up to the top of the bed posts, and then just snap the bedposts (which were very long and thin) by simply pulling her arms together?

a) And how do you think from that position it's possible to raise that shelf and get it over to the phone.
b) There's no way to shimmy a bed that big.

Plugging the phone in and making a call is something she should've done. Not sure why that didn't occur to her. Or at least going to the fridge to get some food.

"When she freed herself, why didn't she try to plug the phone into a charger (there must have been one in the house) instead of getting into the car and driving off. Or take the phone to the car and plug it in there, and call police in that fancy Jag."

Because there was a creep in the house and she wanted to get the fuck out as fast as possible? I guarantee I would have done the same. I wouldn't have spent one more minute in this place.