Hello,
Although I can't answer the specific questions that you asked, I can relate to what you're going through. I'm in the same boat as you. I have a '99 RX-300 with no CD changer. I've been looking for the past few weeks on buying a CD changer and ran across one for sale at http://www.partydown.com/items/item375.htm. They have OEM changers for Lexus at this site. I haven't been able to find any info on the net about this retailer, however. But assuming they're honest, their $300 changer sure beats the dealer's $1000 price tag. Maybe you can look at going this route rather than trying to get the dealer to install a used system for you.
Phatman