Yup, sanding the inside is much less fun than the outside. Luckily, as you pointed out, you don't have to be as exact since you won't see it often.

The secret (as described by both Rob Macks and Nick Schade in their respective instructions) is to use the random orbit sander but tilt it up just enough so that it conforms to the curve of the hull.

As a woodworker the thought of this horrified me since I have been told for years to never tilt since it will leave gouges... and it will if you don't keep the sander moving at a brisk and steady pace.

Start in the middle where the curve is least so that you get the hang of it - by doing this I was able to sand all but the two tips where the sander hit up against the opposite side and wouldn't fit. You are stuck hand sanding here.

You should be able to get a smooth transition between strips. As you guessed, having steps leads to bubbles and problems later when you apply the glass and epoxy (you can usually get them to flatten out but it is a lot more work). The smoother you can get it with 80 grit the easier your job will be later and the less excess epoxy and weight you will use too.

I sanded thru in one spot; after gluing on a wood patch and resanding it smooth you can't detect it from the outside.

Also, if you haven't already found this, the two things that make this much easier are fresh sandpaper (buy in bulk and change often - you can cut faster and control better with fresh paper) and a good sander (I really like my Bosch 3725 - the difference between it and the cheapo Ryobi I had before is hard to believe).