The generated ThunderbirdPortable.exe doesn't pharse any INI setting into the "$SECONDARYLAUNCH" variable, this means it isn't possible to have that exe call "StartProgramAndExit:" meaning ThunderbirdPortable.exe stays open until Thunderbird itself is closed always.
It's not an INI setting. It's an internal variable that the launcher itself uses to track whether this is the second launch of Thunderbird Portable. If it is, it just hands off to thunderbird.exe without doing the splash, adjusting the profile, etc.
Sometimes, the impossible can become possible, if you're awesome!
Ah ok I see now, thanks!