As far as blade performance is concerned, an injector is the same as a DE. A blade goes in new, you use it for X days, it gets blunt, you eject it and inject a fresh one. Injectors are designed to have a blade in them all the time, which has two purposes. Firstly, it keeps the retaining spring(s) in the mechanism at the right tension and, secondly, presents the gap for the new blade to start sliding into. If there's no blade, then the new blade has to be "wangled" into the head, as there's no gap to speak of between the top and base plates. In that case, the blade may ride on the first blade stop, possibly damaging it. I think I'm right in saying that most, if not all, Schick and Pal injectors came with a dummy blade loaded.
The main purpose of the removable baseplates in the S. E. is to allow the user to vary the aggression of the razor. As you have noted, it also allows a blade to be removed without ejection. There are then three possible options.
1- You can then inject a new blade into the empty razor, in which case the problems discussed above may occur. There seems little point in this.
2- You could put the old blade back in, close it all up and inject a new blade.
3- Alternatively, you can eject a blade from the injector into thin air, position it in the razor and close it all up. This rather negates one of the ideas behind the injector, which is to minimise blade handling.
Of course, a lot of this depends on the individual. If you find a plate that suits you, then you'll probably just eject old blade, and inject new one. If you need to clean the head and remove the plate, then likely 2) or 3) above will apply. The same two apply if you swap baseplates.