2 weeks is too short a time to learn how to use a GPS and will probably take the enjoyment away from the ride if you stuff up the routing. Most car GPS can do a route between 2 places, but depending on the model, cannot accept more than one route, you need a dedicated GPS for something like that. Downloading the routes from BBR is a good idea, but you have left yourself too short a time to get it set up right, I know cos it took me ages to do mine before leaving oz last year.
It sounds like the only knowledge about wiring is to directly attach a USB charger to the battery terminals, this is probably OK but not waterproof, so not ideal, plus you would need a mount that is in your eyeline area for safe riding. Plus your TomTom is not waterproof either. It sounds like you should buy and get installed a dedicated GPS designed for a bike and learn it in two weeks. which will probably not happen.
If you have already sorted out the routing in google maps, then print out the turn by turn directions and take them with you(you will only need actual turns and distances to next town). Depending on how many routes you use then that will determine the paper load, but you can dispose of the used as you go.
Take the iPhone as backup with a European data card. so you can navigate your way OUT of your geographically challenged location, if it should happen
Cheers
TS