Update...
Okay, I'm not sure at this point whether this makes me a savvy shopper, or certifiably insane, or maybe both. I went back to Goodwill and bought that nasty Gsix for $20 to get the parts I needed to go on the G4—handle back, headlight bulb, and power cord, plus a plug protector that I'll use another Gsix. At this point, everything is installed, except for the plug protector, which I'll do eventually, and I still may swap the handle back onto my G5 'frankenKirby', which has a charcoal gray one. Given the estimated a la carte cost the parts I've used so far, I'm arguably well ahead of the game and I still have stuff that I could use on other machines in the future; that is, unless I have a further lapse of sanity and decide to spend a ton more money to refurbish it as well.
Meanwhile, I've contacted the eBay seller to request to cancel handle back I ordered last night. The automated response from eBay is that it's too late to cancel the order, so hopefully, the seller will find the message before they send it out, and agree that it would be better to cancel the order and save us both postage by not sending the item out and back for no practical reason.