If humidity in cellar is over 90%, better off in attic. The mulch is not necessary. If care is taken not to puncture the plastic, the boxes should be fine long-term.

It's a bad idea. I'm speaking as someone who is currently trying to deal with a lifetime of odd plans and habits. Can't you get cardboard boxes any time you need them in the event of a move?

Unless your cellar is dry, attic is better. Storing in plastic bags works fine.

I don't think cedar will repel much of anything except moths. I assume what you're concerned with is wood roaches (or any kind of roaches really, they like paper and cardboard), silverfish, etc. If cedar repels them so much the better, I just haven't heard that it does.

I agree plastic bags should be good. If you're in a place that gets hot in summer, the attic would be much less susceptible to bugs than the basement. My attic is 120F when the sun's out, nobody will be living up there. But cardboard can get a little crispy after a few years.