I would echo everyone else's comments about how bad it is to require that the customer do the imposition. I work in prepress, and we hate it when someone not trained in imposition sends us printer's spreads, even when they do it correctly. We're the printers – it's our job. We have the software and experience to do it correctly. I can't imagine a company calling itself a printing company requiring the customers do the imposition. There are many variables that change day to day that come into play with imposition, and it's hard enough keeping up with them when you work in the shop!

Also, they require you to send them native InDesign files?! We only do that when the designer is incompetant or unavailable, and changes need to be made that aren't possible with an exported PDF.

That said, I suggest that you export your InDesign file to PDF, create a new InDesign file (set up as printer's spreads) to act as an imposition file, place the PDF pages in printer's spreads, and send them that. This way, you aren't copying-and-pasting live page elements over and over when there are revisions, you just re-export to PDF and update the links. Your page numbers will be locked down. Fonts and images will be embedded. And if you frequently work on files that are the same number of pages and the same dimensions, you can re-use your imposition file on other jobs. We reuse impositions basically the same way.

Seriously, fire your printer. They may be cheap, but if a job gets spoiled because you messed up the imposition, it won't seem so cheap. The average designer is not trained in imposition, and shouldn't have to be.