MacOS – Can macOS’s ramdisk be swapped to disk by the operating system

You should parse the output of hdiutil, and mount the filesystems you're interested in - because you can't do what you're trying to do with mount.

The first few lines of the man page show supported syntax:

 mount [-adfruvw] [-t lfs | external_type]
 mount [-dfruvw] special | node
 mount [-dfruvw] [-o options] [-t lfs | external_type] special node

Notice for -a, there are no options to specify a node path, which makes your command invalid syntax.

(Also, I'd suggest testing with a proper image file - one that is an actual CD image. Your image is a hard-disk type, with an Apple Partition Map, and no non-HFS partitions. In Disk Utility, choose New -> Blank Image and note all the selections carefully. You want "CD/DVD"; you can then edit the partition map after you've created it; say, to change to two partitions.)

Your question contains the most important thing needed to secure a computer against a motivated attack to compromise a FileVault 2 protected Mac volume.

1. Don't connect FireWire to a device you don't or can't trust while you are logged in to an account that has file vault keys active.
2. Pick good single use passwords to reduce the chance of other compromises degrading the security of your FileVault password.
3. Update to 10.7.3 and verify sudo pmset -a destroyfvkeyonstandby 1 hibernatemode 25 power management settings that force hibernate mode to secure your keys from compromise when the device would normally "sleep"

I do follow Rich Trouton to keep up to date on his blog for nice commentary on securing macs. The mix of up to date topics seasoned with experience as a real world system administrator make his writing very valuable to me.

The crux of the issue is your parsing What user behavior is necessary to have security. I always like to think of security as a mindset and ongoing attempt to plan, implement, measure and adapt. Security isn't something you buy or something you "set up" and training users to actually not divulge the passcode they have used to store their unlock phrase is the weakest part of FileVault's security layer. Not reusing that passcode - having a system where you get your users to understand why their keychain password needs to be unique and secure is far, far harder and takes far longer than just setting up a plan for implementing file vault initially. Best of luck in your quest for security!