Well, it looks like the brown tabby is finally turning the corner. He's not eating on his own yet (that I've observed), but his appetite appears to be returning and his diarrhea is coming under control. I'm still forcing food/fluid 4x or more times per day, along with Vitamite II. The tiny fraction of a loperamide HCl tablet seems to have pretty done the trick (per online veterinary advice), in terms of slowing down the process of food through his gut, and reducing dehydration. (Trust me, I never would have administered this medication unless I found more than a couple of veterinarians that recommend it, nor if I couldn't find dosage information.)
Names under consideration for the brown tabby, chosen strictly for meaning:
Varro - durable, strong
Tiergan - strong-willed
Faustus - lucky
Ellis - Pronunciation of L.S.; stands for Long Shot
The black & white kitten is eating and drinking entirely on his own. I'm supplementing with some Vitamite daily, but that's about it. He already feels like he weighs twice as much as when he arrived! Still bony, but a little less so every day. His back end is still wobbly, but that's to be expected. He tends not to walk so much as slink along, belly fairly low to the ground. Poor guy is still so scared of me!
Still thinking on a possible name for him.