Imagine that they make a mistake and they send me all that coins.
If they sent you a suitcase of cash you could spend it but bitcoin is traceable. Shortly after you set up an account somewhere to spend it you would get a knock on the door. I doubt you could even move very much of them on localbitcoins before you're sitting across a coffee shop table from a fed trying to buy them from you. One of the great features of Bitcoin is the permanent record.
This is technically true, however there are a number of mixing services available that could help you disconnect the coins you are spending and the coins that were sent to you in error. Another issue is the fact that there are no laws against receiving funds at your address and spending them. As long as you would not steal the coins from the government they would likely have no case against you.
They can always find a case against you. Federal laws and regulations are so vast and unreadable that most average people commit crimes everyday in their everyday life. The key is to avoid their radar and not become a target.
This may be true, assuming that they can find you. If they simply send coins to an address that you control that is not linked to you then you could simply mix the coins sufficiently so that when you go to spend the coins no one will know they are the same coins that were sent to you in error. If they don't know who to harass then they cannot charge you with bs crimes
