Do anything with very rare items

There is no reason for bots to actually do that since they're specifically designed to play along with you. So a bot sitting in the goal box and not going after the nearby ball is definitely a hangup in the bot's path logic or something related to that.

It's very likely that Psyonix's AI update set some things back. This happens to pretty much everyone designing AI because there are so many factors that go into intelligent play.

Yes, according to this reddit thread, both your ranks and items are stored locally.

Your save file should be located in a folder inside \Documents\My Games\Rocket League\TAGame\SaveData\. Deleting it will reset your progress.

However, if you have Steam cloud synchronization on, you will just download the save from the cloud, so you either need to turn the cloud synchronization off or forcefully overwrite the cloud contents (see here for an extensive description on how to do so).

Your DLC items should still be available (but make a backup before trying), since it acts the same as if you bought the game with all DLC and started it for the first time.