There is no copy protection in System 7.5.x. Even in the era of its original release, Apple distributed it for free on the Internet.
The Apple Installer 4.0.3 Error Guide p.10 gives this description for the "The installer document [...] is unusable for installation" error message:
This error occurs when the Installer is unable to find a required
resource specified in the installer script - 'inrl', 'inpk', 'infa',
etc., has become corrupted, or the resource map has become corrupted.
This error has also been known to occur due to the following reasons.
- Two packages are included for installation which involve conflicting actions. One package calls for the installation of a file
or resource, and the other package calls for it's deletion, if it
- The same source resource is specified for installation as two different resources under separate resource ID's. In other words,
there are 2 'inra' resource which reference the same source resource,
but specify different target resources.
From the description, it sounds like the several possible causes can lead to the error, maybe corruption of the Installer Document itself (in the Installer-related resources), or maybe corruption of some other resource or file they reference, besides the numbered installer design errors noted.
The most obvious cause would be data corruption, either of the disk itself, or due to some hardware problem with the machine that causes it not to be able to read the disk correctly (malfunctioning floppy drive, logic board, RAM, etc.)
inrl resource contains "Installer Rules", logic which is specified in a kind of scripting language and the installer will use to implement automated behaviour such as for Easy Install and which can include actions for specific machines. (See Apple Installer 4.0.8. Technical Guide, p.80).
This is speculation, but due to the parts of the installer that this error is related to being possibly machine specific, maybe there is corruption in a part that just isn't used in the installation on the Performa 460 and so didn't come up there.
Someone who has the capability to figure out the details of the Installer Rules from the binary compiled versions in the
inrl resources may be able to shed some light on what's present on the IIsi and absent on the Performa 460 or vice versa.