Here are the answers to all your questions
1) Is it possible to assign access pass/Token to AD group? If yes then how?
Yes absolutely. Make use the custom properties in the QMC to create a group and assign with this custom property.
Then create a rule and assign token to this.
Check the screen shots:
2) Is it possible to add users in QMC without asking them to login the portal first time?If yes then how?
Yes, you will have to sync the active directory and you will have all the users in your active directory synced into the management console. You can then assign tokens, create groups etc ...
3) What's the benefit of configuring AD link and syncing it in QS QMC? I was assuming that if we add AD link in user directory connections then we will see all the users/groups in the "License and tokens" section so that we can assign license to them directly from there
The answer for the second question should answer some of this. Also, you will have more control over the users if you sync them.