NSBundle and CFBundle aggressively cache the contents of application bundles to improve performance. Although you edited the Info.plist, the Finder and Launch Services don't yet know that the application is any different.
Running the following command in Terminal to touch the application bundle will force the Finder (and Launch Services) to re-register the application, thereby noticing the changes you made to the Info.plist:
touch "/Applications/My Cool App.app"
After running that, deselect the application, then reselect it in the Finder, and then try launching it.
Note that this won't necessarily guarantee that you'll be able to launch the app. The developer would need to make sure they set a deployment target of 10.4 and/or use weak framework linking to assure that the app will be fully backward-compatible.
The only way to run multiple, separate instances of Terminal is by launching them with the command line open -n -a "Terminal" -- understand that this isn't a great way to run things that aren't meant to be run like this. There are shared preferences and plist files being used by these instances so you may run in to trouble.
The reason Fluid works so well is each app is a separate WebKit browser instance, with a unique preference namespace, and without the normal browser window decorations. It's a heavy hack to say the least.
You can script Terminal with AppleScript and then save the AppleScript as an application. This script forces a new instance of Terminal to the foreground and then runs an ssh command in it:
do shell script "open -n -a Terminal"
tell application "Terminal"
set currentTab to do script ("ssh user@server;")
# Additional commands can be sent using...
# delay 6
# do script ("do something remote") in currentTab
end tell
Enter that in to the AppleScript Editor, customize it as suits your needs, and then select File > Save... and where it says Type: in the Save dialog select Application.
This will create an unsigned .app file you can distribute as an Application to start ssh sessions. If you have a developer certificate and want to code sign the .app so users don't get a warning about running untrusted code use the File > Export... menu option, select Application for the Type and make sure the Code Sign: option in the dialog pointing to a developer certificate instead of Don't Code Sign.
It is preferred to code sign the app if you're going to give the app to other users.
If you run that, you'll get entirely new Terminals started, independent of each other, that you can Cmd-Q quit without quitting the other instances. Replace Terminal in the script with iTerm if you prefer it for your command line work.
Best Answer
You could also use Fluid to create a site-specific browser. SSBs often feel more like restricted versions of browsers than standalone applications though.
One example of a Fluid application is https://github.com/drdrang/drtwoot.