Mutation testing tells you the gate's tests are strong. It can't tell you the gate sees every way a command reaches a closed drive, because each mutation only breaks a condition the gate already has.
The gap I'd test next is indirection. A gate that reads command text sees python fix.py, not the open("D:/...") inside the file the session wrote a minute earlier. Windows has a quieter one: mklink /J C:\work\d D:\ creates a junction on C: that points at D:, needs no admin rights, and every path through it reads as a C: path to a text filter. I'd add a few of these as tests that are expected to get through today, so the known gaps are written down instead of assumed closed.
For the vaults and the backup clone, NTFS can hold the line without reading any text. Running the sessions under a separate standard account, with deny ACLs on those volumes, turns the most important part of your list into something the operating system enforces. The gate then keeps the job you already give it: catching a session's mistakes.