The vet gave her a bcs of a 3 out of 9. So, that tells me weight gain is in order.
There are a range of different scales for bcs, so it's a bit ambiguous, but I'm thinking a 3, especially if it took the vet to tell you that, means your horse is not much under at all?
I'm guessing if you've had the vet to her & he commented on weight, he's probably checked her teeth & discussed effective worming? Ulcers are another possibility which can cause weightloss/hard keeping despite adequate feed. Also you don't mention nutritional supplementation and imbalance/deficiencies can also mean horses with adequate feed still aren't healthy.
husband believes in the philosophy of the less grain the better.
Tend to agree with that. I don't think grain is necessarily bad for horses, but the high starch content, along with not being easily digestible - unless it's cooked or otherwise processed well - mean that I think it's best avoided if possible. Instead of grain, beet pulp, soy hulls, lucerne/alfalfa, oil(very small amounts) can be added to other forage when more calories are needed.
She gets about 2 lbs of a 10% horse/mule feed, about a pound of stabilized flax meal, and all the good quality grass hay she can stand.
Free choice hay/forage is great. If you would like a specific opinion on the horse/mule food, you need to tell us what it is. Starchy/sugary diets can actually cause weightloss, due to upsetting the gut. It's also important that feed is given little & often, so I'd be feeding that amount of hard feed over 3 or so meals daily if possible, which if high starch, will also help reduce the 'side effects'. A pound of flax sounds like a huge amount to me. Linseed/flax is great IMO, full of omega 3 & other essential fatties, but I think of a half cupful as a large amount. Might be worth checking that one with a nutritionist. Linseed also loses it's goodness very quickly with processing, so I feed whole seeds which I grind fresh daily. I don't know whether 'stabilised' means it keeps all it's nutrition, or it's just not so depleted as otherwise. Another question for a nutritionist.... feedxl.com is great.
Im thinking the 10% grain needs to be bumped up to a 12 or 14% grain. Im also thinking about adding soaked alfalfa cubes or beet pulp.
I don't know about protein levels, but as above, I think if you do need it, it's best to find healthier sources than grain too, and we can't give you an opinion on the feed when you only give a protein % - could be anything. Alfalfa & beet pulp are both high in protein, along with calories & low in sugar/starch, so I think these are great choices.