Hi Kiki,
welcome back on the Bude.
Hope you have the same fun as before
To answer your questions:
1. there are plugins for GTL on GTR4u or NoGrip to have other starting countdowns. Just look around a little bit.
2. Start your "Autoupdater" and go on "Tweaks". At your profile setting, under "PLR-Setting, you can find the options "No AI Control". Activate this option and you can make your own decisions after passing the finish line
3. you are not able to race only with mod-cars. They are always integrated in other classes or are bound to other cars.
The only way to make it possible is to make a new GTL-Intallation and just install the cars you wish to drive (its an easy way, too)
GTL disables tho possibility to make new classes or integrate new cars for itself. So we have to live with the situation.
Cheer and great fun for the future

,
Ron