I just got in from the barn, the rain has stopped! By nothing short of a miracle, the arena is basically dry, so I put the lungeline on Scout and gave him a little exercise. Between the rain and a now fully healed saddle sore, he hasn't been ridden in a while, but I may saddle up tomorrow. Scout lunged beautifully, he was relaxed and calm, holding his head nearly on the vertical (a smidge ahead). I warmed him up well at the walk and trot, lots of transitions. I cantered him once on the right lead (his good one), he took off immediately on cue and did two laps of good, even paced canter without hauling on the rope, then came down to a trot on cue. I turned him around and tried the left lead and he was up to his same old tricks. I brought him back down to a trot to regroup when he took the wrong lead as quickly as possible (he has a lot of go), and asked again. He took the wrong lead for half a stride before he swapped. I cued for a downshift and we regrouped again. The third time he took off on the correct lead and held it for about six strides! I immediately praised him and allowed him to stop and rest. I sent him out to the left again, did a lap of trot and cued for canter. Again, a little rough on the transition (two strides of fast trot), but he took the correct lead again! We did two more trot/canter transitions to the left before we cooled down. Hopefully I can ride tomorrow, and we'll see if Scout's lightbulb moment carries over, lol.
Thank you all so much for your input! I'll get another update up when we ride!