Curtis,
Man, I must say that when you ask questions you really get into the heart of the matter!
More or less in order, the first thing to remember is that the basic Rebreather is really a simple system. You have a bag, or bags to breathe in and out of, a cannister of kitty litter, and some way to supply oxygen.
Once you get deeper than 20 fsw or so, you have to add a diluent supply. As far as it goes, that's it, until you start getting fancy.
Now you have a sensor system, usually three, and a logic circuit that polls the sensors, ignores the out of range signal (if any), and sends a signal to the solenoid valve to squirt some O2 into the system. It's kind of hard to drift away from that basic model. (I'll let the designers and engineers argue the merits of which logic model to use!
)
Now, when you add a deco function to your computer, you get into arguments over which algorithm is "best" for doing the job.
Nevertheless, the APD Vision, the Shearwater, and the HammerHead are all excellent computers offering wide feature-sets, and great performance. Unfortunately, due to differing fittings, cannister sizes, and mounting points, they are not interchangeable.
The electronic control systems are semi-independant of the mechanical systems. If the solenoid fails, the needed gas can be added manually and still read on the displays.
The H-S Explorer, and the VR-3 are excellent deco computers in themselves and can operate in the stand-alone mode, but are best used with a link to an independant O2 sensor. You can, as you say, fly the unit manually on the one sensor and manual addition if the main system fails. They do not have any connection to the gas addition systems.
Hope that helps.
Cheers!
Rob