Congratulations on starting with sheep.

Firstly, the store lambs should be wethers. Gimmer lambs could come a-cycling at 5 months old and, even if it didn't bother you, it would bother the mart and/or slaughterhouse staff if they went for slaughter and were found to be in lamb.
Yes the Crystalyx tubs will get water on them. Usually a little doesn't matter and they'll lick it off; in extreme conditions you may decide to tip some water off the top now and again, and yes, you will lose some of the minerals you've paid for that way. If you can make a hole in the tub just above the current level, you'll save a bit. Or of course site it out of the rain, if you have such a place in your pastures!