(Like you, I am making informed guesses on how things may be done in FileVault.)
If you take a machine, such that, upon boot, typing a given password is sufficient to unlock the contents, without any intervention from external systems (i.e. it also works without a network), then, by definition, that password is sufficient to recover any encryption key and, ultimately, the data. Therefore, if at some point there are two passwords which are sufficient to boot, then the security cannot be stronger than that offered by the weaker of the two passwords. If one of these passwords is "test", then security is not high...
This might be recoverable by deleting the data elements related to the weak password. The conceptual mechanism is that the encryption key for the disk is encrypted, with a symmetric encryption algorithm, using as key something which is derived deterministically from the password (with a Key Derivation Function). Say, the disk encryption key is K, and the recovery partition contains EK1(K), EK2(K),... for keys K1, K2,... derived from the various passwords which can unlock the disk. There can be extra layers of indirection.
If password 1 is weak (key K1 is derived from a "guessable" password), then we want to prevent the attacker from learning EK1(K). Deleting that value ought to be enough, because the attack model is that the attacker, as some time T, steals the hardware altogether, and thus learns all the hard disk contents at that moment, but he has no knowledge of previous contents. This is subject to the following caveats:
Whether the values for account 1, when access is disabled, gets deleted, is unclear. I don't know the answer for that. It is not easily tested (you could try to reenable access for account 1 and see if typing the password for account 1 is needed for that, but even if it is not, it does not necessarily imply that the EK1(K) was kept around, because asymmetric encryption could be used).
Whether deletion from a disk really erases the data is a semi-open question. Most filesystems, when a file is deleted, don't overwrite the file contents; they just mark the space as reusable. When the file is specifically overwritten, whether the data really dies is debated, and the answer is likely to be different for a SSD.
FileVault 2 apparently has an option to store the encryption key (or something equivalent) on Apple's servers, which opens a whole range of new questions about the abilities which are thus conferred to Apple.
To sum up, it seems safer to assume that the data confidentiality offered by FileVault will be no stronger than the weakest of all passwords for all users who ever had access to the disk.