Hello!
The other questions are answered in the ToS, specification page and Privacy Notice, therefore we invite you to read those documents, further clarifications may come from the community or from us if necessary: https://airvpn.org/tos The Terms of Service https://airvpn.org/privacy The privacy notice and terms, scroll down for additional safety measures according to best practices as well as GDPR prescriptions https://airvpn.org/specs Specs overview
In AirVPN, the problem you mention is not related to USB specifically, because USB support is disabled on the kernel of our servers and any reboot to make the server re-start to run a different kernel in order to plug secretly USB devices will cause the server to be rejected by the infrastructure, but adversaries don't need to plug in USB peripherals. A more effective attack comes from outside the server and a defense against this attack is not possible on the server itself (simply because the adversary does not interfere with or touch the server), it must come from a pro-active action by the user. Please see here: https://airvpn.org/forums/topic/57163-pen-register-connection-logging-on-airvpn-server-janfeb-2020/
Kind regards