If you buy a splitter that also has an extension wire on it, you should be able to tuck the splitter itself and most of the wiring out of sight. If you don't plan to use the cigar adapters in other cars, then you could tuck those out of sight too. If you're going to do this, then I'd recommend taping the cigar plugs into the adapter quite firmly, as they tend to work loose through the normal vibration of the car. Not a problem if they're right there in front of you, but a pain if they're tucked away in the footwell or under the seat.
Whatever arrangement you come up with, make sure it allows you to hide the TomTom bracket whenever you leave the car unattended. If the crooks see it, they'll often break in on the offchance that you've stuffed the TT in your glovebox or under your seat. Whether you've taken your TT with you or not, you've still got a broken window or busted door lock to sort out
The Lithium Ion battery thing is a bit of a pain, but they're still the best affordable technology for portable devices when it comes to power for size and weight. Don't be afraid to use it on battery - that's what it's there for, but if you routinely use these items where there's an alternative source of power, then use that instead.
Deterioration really isn't that rapid, but there was someone on one of the TomTom forums whose GO has next to no battery life left, thanks to him regularly charging it at home and running it from battery in the car. This was a couple of years into ownership mind you, and some scare stories say that the lifespan of a Lithium battery is about three years from date of manufacture anyway (and you don't know how long it's been on the shelf when you buy it.)