Grand Theft Auto Online – Fastest SUV in GTA Online

Since i finished all the elite challenges yesterday, but didn't do the fleeca job with the same guy (picked a random I've never met), i can say with a probability of 99,9% that the fleeca job does not count into the heist challenges.

Based on what you've described, it is likely that the online lobby you were in had a hacker/modder. These people are able to do a lot of crazy stuff, including spawning weapons that you can pick up.

These weapons that you've picked up should not persist between sessions, so re-starting the game or entering a new lobby should remove them from your inventory.

Rockstar has, thus far, been largely ineffective at stopping many of these hackers. But if you manage to capture video proof of someone hacking (often difficult unless they're being obvious), you should submit it to Rockstar support by opening a new support request.