At 16 or 17 she could definitely have the start of Cushings, or something else that's not age related. Yup SmartPak has several different options, and there are plenty of non SmartPak options too. Most you would want the extra supplement of Mg with.
Do run whatever you choose by your vet, as mentioned it's area specific and what might be optimal for me could be flat out dangerous for your mare (the Selenium like I mentioned for example).
I would go with beet pulp first, but no need to switch to beet pulp as you already have the alfalfa- do feed the bare minimum though. A small handful of pellets soaked would be more then enough. Honestly if you get the supplements in pellet form then you probably won't even need the alfalfa, I bet she'd just eat it plain!
If you get her to loose too much weight on this diet and need her to gain then I would get some beet pulp, but until/if then there's no reason to worry about it, you can at least use up the bag. Remember you aren't actually feeding it just giving just enough to get the supplements in as she doesn't need anything more.