You want a reining horse to be set low in their hocks...makes for an easier stop. While she's still growing, it does appear that her hocks are in a good place to help her with this.
You also don't want a reining horse to be too wide in their shoulders - obviously, this is something you have to see from either on the horse or from another photo angle.
Since she's still young, I wouldn't worry too much about the downhill build for now...she should balance out when she's done growing. She does seem a little chunky though
Working her will definitely help her muscle tone.
There's absolutely nothing wrong with working her for reining. I like for a horse to be handy, and reining teaches a lot of that. Once you back her and get her going good, you will probably have a better feel for how she will take to the training. If she stops with her back hollowed out and her nose up in the air, the stops are going to be hard on her. Doesn't mean that she can't do it, but it does mean that she'll have to work harder (which over time can lead to joint issues, etc.). She'll need to have a nice rounded canter, and be able to support herself on her hind end (and not strung out on the forehand like so many ponies are). Also (and perhaps most importantly), she needs to have a level head about the whole thing. If she gets hotter as she's ridden and put through the manuvers, she's not going to make a good reiner. Cool and collected is how you want them to be through the whole thing :)
She's a right cute mare - Good Luck!