1.5.0.2 Plugins is not wired up properly

It looks like you did not update the .nsi for the pluginsdirectory new location?

PluginsEnvironment:
;=== Set the plugins directory if we have a path
StrCmp $PLUGINSDIRECTORY "" UserProfileEnvironment
IfFileExists "$PLUGINSDIRECTORY\*.*" "" UserProfileEnvironment
System::Call 'Kernel32::SetEnvironmentVariableA(t, t) i("MOZ_PLUGIN_PATH", "$EXEDIR\$PLUGINSDIRECTORY").r0'

Would tell me that you are only looking in $EXEDIR\$PLUGINSDIRECTORY ?
and seems it may be already set to StrCpy "$PLUGINSDIRECTORY" "$EXEDIR\Data\plugins" earlier w/o any INI file?
Moving it to your default settings Data\plugins says i need to install some I already have??