As much as I don’t want to admit that the project is truly dead, I think I’m indeed going to have to put the Kart on permanent hiatus. I’ve just got too many other things going on right now to manage or contribute to the development, and it sounds like there are some great alternatives. Moltin and Perch look like the options I would personally pick.
There would still be some major advantages to having a shopping cart integrated directly with Kirby, so maybe soon I can get a client to pay for my hours while I work. 
But for now, it’s sadly just not financially practical for me. I thank everyone for their input so far though, especially @distantnative and @BenSeitz, your insights have been incredible valuable.