Tom Tom user for many years and happy.
I think one other thing you could try is to rename the last/latest backup that is on the computer. I have a guess that is one way TT knows if you have completed the update. (I could be wrong, but give it a try).
By the way, the backup is how you should be able to restore to the one you had prior to the update.
Another thing when doing an update, be sure that the screen saver and sleep mode are turned off.
Now to the problem about the number of hours for the install. What does TT say when you contacted them?
TomTom comes out with a full update every 3 months. After that, they have small updates which I never do.