1. I appreciate the brainstorming on ways to do this, although I still think you should just ride it and see what happens. That spacer isn't designed to retain the spindle position, but you are correct that it absolutely would do the job.
2. Increased roughness between two surfaces under a clamp load often helps retention, but only if the roughness is on the harder surface. In this case it doesn't have much retention force to begin with and the spacer is the softer item so it won't do much. Removing material will in this case loosen any fit, especially if you end up where the bearings ride. A hose clamp (ie something that can be made tight, yet is removable) around that center spacer would probably do the trick if it cleared the BB shell. On the surface that seems like an ugly hack, but not that much different than a proper shaft collar at the end of the day. Bonus points because unlike Loctite it is much more easily reversed or adjusted.
3. In general, be careful how you press on bearings. You always want support behind the race that is being pressed so that the load path does not travel through the rolling element during installation.
4. I'd re-use the bearings if they feel good, but as noted they don't cost that much.
5. There's something odd where the bearing sits on the right side of your last picture. Is that corrosion, or just grease from disassembly?
6. I live in San Francisco, but will be spending several days in Holland next month. If you're willing to make the short drive over I'll bring a micrometer along and we can find out if the spindle is actually undersized or not, and by how much. The purpose of that portion of our trip isn't primarily to go riding, but we will probably have our tandem with us as well.