I have a few questions about Security and Veracrypt and hope you can clarify it for me.

1. I don't want the VeraCrypt hidden volume I created to be damaged. If I mount the hidden volume and transfer files there'd no damage, and if I enable the option 'Protect hidden volume against damage caused by writing to outer volume ' In the Mount Options dialog window, and write to the outer volume, there is no damage to the hidden volume, is that correct?
2. if someone has access to a Veracrypt encrypted flash drive, they wouldn't be able to mount or write information to it, and BadUSB, flashing malware into the firmware wouldn't be possible?
3. The only possibility would be to physically open up the flash drive and insert something into it? How likely is that to happen by someone who isn't the government and how much it would cost?
4. I assume that it would be different for a PC with an encrypted hard drive as they would be able to flash malware into the firmware if the BIOS isn't password protected, but what if the BIOS is password protected but you can still use the multi device boot option and boot a Linux operating system from a flash drive? (I know that PC does that). Does that mean that they could flash something into the firmware? How do I test if it's possible on that PC?
5. If four isn't possible, the only possibility would be again to open up the PC and insert something into the hardware, is that correct? How likely is that to happen by someone who isn't the government and how much would it cost?
6. Is there really no way to check if they inserted something into the firmware or to check if they inserted something into the hardware? How do you keep your information safe if your PC or flash drive could be compromised with no way to check if it has been compromised, aside from getting a new machine?

Please answer as much of it as you can, it's alright if you don't know all of the answers. Thanks