Since February 1st we have been feeding her once a day, 2 Quarts of sweet feed and weight builder
I do not like sweetfeed. Akin to feeding your kids Macca's every day IMO. Low NSC feed - that is, low starch/sugar/grain is best for horses generally.
Horses are built to process little, frequent & low starch feed, so it's always best to feed hard feed little & often too, rather than just 1-2 larger meals daily. This is particularly important if you're feeding rich, starchy feed, to minimise the problems it can cause. As you're feeding a weight builder, I gather she's a 'hard keeper'? One of the problems starch overload can cause - especially when not fed little & often - is weightloss, or failure to thrive. So, especially if you can't feed little & often, I'd ditch the sweetfeed. You may then find you don't need the weight builder either.
2. How much should I feed her?
Free choice, or 2.5% bwt hay, a nutritional supp/ration balancer and if/when she needs extra - say when she's in hard work - feed her hard feed, as needed.
3. How many times a day?
As many as you can manage. I know we're all in the 'real world' & it may not be possible to feed more than once or twice, but if so, keep them very small, low NSC & I wouldn't feed things like oil unless I could feed at least twice daily.
4. Should I slowly switch from sweet feed to the new feed?
Always. But depending on her condition, etc, you may not need any more than hay, aside from a nutritional supp.
5. How much hay should I give her a day? (I was thinking of putting a few slow hay net feeder in the dry lot and one or two in her stall at night)
A slow feeder is a good move. Horses need at least 1.5% bwt daily in roughage. If you're wanting her to gain, I'd work on 2.5%