1) Yes. Shimano cassettes always have a built-in spacer on the top cog and I don't think any of the Shimano 9-speed cassettes use a 15 that has built-in spacer. The reason a built-in spacer is required is so that the cog will engage enough of the freehub splines. This is especially true if the freehub is a "compact" type where the splines don't go all the way to the end. (http://www.sheldonbrown.com/k7.html#hyperdrivec). But even if your freehub is not compact, I don't think it'd be wise to put on a top cog that doesn't have a built-in spacer, especially on a tandem where there are two motors generating torque. I believe the largest top cog in Shimano 9sp cassettes is a 14. However, the two 9sp cassettes I could find on techdocs.shimano.com (Ultegra http://techdocs.shimano.com/media/te...9830611831.pdf and Tiagra
http://techdocs.shimano.com/media/te...9830608763.pdf) show that many of the cogs come on carriers and cogs on carriers must be added and removed as a unit - i.e. you are very limited in customizing. I thought that the 105 9sp cassette didn't use cog carriers but that's not on techdocs and I don't happen to own a 105 9sp cassette so I don't know for sure.

2) Yes. The difficult part will be finding cassettes and cogs that will work and give you what you want.

3) Yes. HyperGlide cassettes are designed to be used as a unit and the 15 that is part of a 12-23 is different than the 15 that is part of a 12-27. However, there's usually not a noticable difference when you mix and match between different range cassettes. I've done it plenty of time.