Indeed, the type of handbike pictured isn't as stable as some other models, but I anticipate this setup to actually work quite well. As you can see in the picture, the manufacturer(s) of these things weren't too concerned about head tube angle/rake/trail. There is quite a bit of trail in the design, so the steering should be plenty stable at speed. Add to that the fact that the steering wheel is also being driven, and it should be just fine at 20 mph. Under electric power, I imagine that I'd be cruising between 15 and 20 mph. 25 would probably be where I'd start to leave the comfort zone (mainly because all braking will be done on the front wheel). In all likelihood, I'll probably add a disc tab onto the bike at some point (as the fork is made of steel). So, the braking capabilities should be greatly improved, too. Having handlebars in place of the cranks should make should make for a fairly stable platform.
As for the rear axles... Some of them are a quick-release mechanism, but I will be using a type that threads into a receiver (far stiffer and more secure). I look forward to getting this thing going. I just want to make sure that, when it comes time to spend money, I will be investing in quality equipment and batteries that I can rely on. I've wanted to get into alternative transportation for a while now, and this might be just the ticket. I considered converting my truck to run on vegetable oil, but that setup is extremely messy and isn't good for short trips around town (and still has me driving a huge pickup). An EV conversion would be extremely expensive and time-consuming. However, this project looks like it would be able to cover 90% of my daily commuting needs. Here in Albuquerque, there are bike paths all over the place, and the weather is rideable all year long. I'm excited to get this thing going! Thank you for any advice that you are able to offer.