Sorry about your bad experience with TT Support. I am an authorized reseller of TomTom and I will try to answer as much as we could:
1) New features added in TT6 but the list may not be very long. The most benefit you could potentially enjoy is the new map data updated from their first edition of merging TeleAtlas map data with GDT (a US company they acquired a few years ago).
For complete list of new features, please see our product page using the link below and click on the Features tab:
TomTom NAVIGATOR 6 USA & Canada Software & maps for PocketPC, Palm, Nokia and Smartphone(Add to Cart to view Discount)
2) There is a lot of confusions on this. There is no straight answers to this. Why? Check out the PocketGPSworld.com TomTom Compatibility Matrix they have put together:
TomTom Compatibility matrix for program builds and map versions and the 10-page discussion on this topic (link located on the bottom of that page)
3) TomTom 6 tends to render / re-draw maps a little bit slower (not talking about significantly slower such as longer than 1 or 2 seconds and not completely un-usable). Not sure why and I cannot offer the details explaination to this reason. Can you combine TomTom 5 you currently have with the latest maps, the answer is yes and no.
Yes if you purchase TomTom Maps of Western Europe, SKU 458:
Maps of Western Europe
OR
If you have upgraded to TomTom 6, you will need "omTom Maps Collection Western Europe - DVD ", SKU 708 instead:
TomTom Maps Collection Western Europe - DVD (compatible with TomTom Navigator 6 North America, GO series, ONE, Rider)
NOTE: Alternatively, you may purchase TomTom 6 upgrade from TomTom.com. Once you have installed this version 6, you will get a TomTom HOME, a PC program. With this program, you may purchase a list of latest maps compatible with your TomTom 6 North America Upgrade package.
4) Cannot comment on this potential issue on their e-commerce website. Our website will definately accept your order.
Hope this helps.