So I stopped by the farm on my way home from work today (yep, half day today! Woohoo!) and saw that the WM was more prevalent than I thought. I suspect some of it may have been knocked down temporarily from the heavy isolated rains we've had the last few days, but it's definitely still less than it was before. I will be applying another dose late afternoon today and will go with the heavier concentration I used last time.
Greg, thanks for clarifying that diquat is contact only and isn't likely to have a cumulative effect. Didn't realize that. I'll probably be picking up some Whitecap from you as I'm still not ruling out that path. At this point I don't care about the submerged stuff so much as I did when I first began this - my goal is purely to eliminate the WM at just about whatever cost, so if I lose other plants, so be it. I'll probably hold off till spring for that.
In any event, I have made progress for sure and will keep after it.