Mare won't eat her supplements/will this go bad fast?

If its powder try putting the powder in a plastic bag with some cut up apples and wet fruits at feeding time and shake it up so it sticks to the fruit if your horse likes apples she will definitely eat it. Or wet some carrots and do the same. If she still doesn't eat it, add more fruit to the powder, it'll make a less concentration and harder to notice. Goodluck!

I definitely would not wet them days in advance (or even hours). That's something that should be done at feeding time. If you board her, and you're concerned about them not wetting it, leave them water bottles (or whatever oil you use) to keep in the feed room to make it easier on whoever is feeding. If they won't take the time to wet it for you, they need a new job ;)

Just as a side, since it was brought up, and not to judge you or try to convince you to change... I don't like Strategy, either. I've been managing barns for years, and NEVER had a horse colic on my watch until I moved to a barn that feeds Strategy. Now I've got colics left, right, and centre because the barn owner is adamant about what she feeds them (Strategy with a little Omelene... gag on the last bit!). She even gets her hay from the same man the rest of "my" barns do. The only difference is the grain. And there was a study done years ago that showed a higher incident of colic in horses that eat Strategy.

Four days should be okay, but no longer. Electrolytes and Probiotics work to keep the horses digestive system on track, and without these for an extended period of time you'll have a mare with an upset stomach and runny bowel movements. You should be okay, and think about switching feed, I don't like Purina Strategy, think about doing your research on new food so you won't need those Electrolytes and Probiotics because healthy mares shouldn't have to have those if their normal everyday diet is good. I hope this helps!

No, you shouldn't wet them days in advance - it will get nasty. Will the barn wet it for you when feeding? I make up my horses' morning feed the night before but leave the old guy's dry until I feed it. My son fed one evening and pre-wet the feed, and by the next morning it had a sour, fermenting smell to it already. I think you could get away with some pre-wetting in the wintertime, but definitely not now.

I wouldn't wet them until just before use. It would encourage bacteria/mold growth.