> [..] but then a logical disconnect kicked in.
That's weird, Paul.. are you sure that you specified a valid database on the command line (within quotes if necessary) and that TP had read/write access to it?
It seems to work fine here. I have created two shortcuts that each point to a different database. They work as expected regardless of the order in which I launch them, and that's with a thumbs.ini that specifies a different startup database.
However, in the [Startup] section of my Thumbs9.ini file, I also notice this setting:
SaveStartupDB=0
..the name of which suggests that the currently used database won't be saved as the default startup database for the next time (I guess that's how the 'last used' option in Preferences | Startup was implemented.. if 'on', the value here would be '1'). Anyway, as said, my TP9 is configured to open a specific database at startup, and a database specified on the command line seems to override that without problem (I've tried this with build 3936 of early April).
As for the idea of multiple installations, that might not be possible due to a registry key that defines the folder where TP looks for its INI file (and so, you would still have only one .ini file). Another option might be to write four batchfiles, each of which copies one of 4 different ini files (each containing a different startup database setting) to thumbs9.ini and then launches TP. A drawback of that approach would be that any changes that you make to your preferences won't be preserved because the thumbs9.ini gets overwritten upon the next program launch. Still, from what my little experiments show, it shouldn't be necessary to resort to such an exotic solution.