One other idea.
The ViaMichelin website now has all of the routes available as a .gpx file.
I don't have the webaddress at the moment. But, from memory, it's where they hide away the links PDF versions of the routes
You should be able to download the .gpx to your PC and (I think) open it up in TomTom straight away.
Thinking about it again, one of the reasons why you may be getting strange 'offshoots' is that the Mpasource maps and the TomTom maps may not match exactly. What I think Tyre does is convert a standard Mapsource file (readable by Garmin) into a .gpx file. It then tries its best to convert it into a file format that is readable by a TomTom. Two or three stages of conversion. If the via point I created was not exactly on the same road in TomTom, then the conversion may well do its best to join it up with others that are.
Don't forget that these are often big routes of several hundred miles, sometimes down smallish roads, it's easy for an error to creep in.
Rather like a bod giving you instructions to their house: "Well, you go down the road for a mile or so, then across the mini-roundabout, then right at the lights... no wait a minute, right at the lights, THEN across the miniroundabaout... on for about a mile, you should see I think it's Tesco on the right, No? Ah, she tells me it's the post office... anyway, before I was interrupted, you carry on for about a mile or so and... oh, I'm sure you'll find us.... bye" None of which you have listened to anyway