1. Put all your code/events that are running too early in their own group set deactivated by default. Activate the group inside the button that launches game when character selection is finished.
2. Second option is set your events like move/spawn, etc to condition out true a global like layoutstarted=1 and set that global as mentioned above (I do this in my space shooter in my sig).
3. Lastly you might have to reorganize your events so the character selection stuff does not need the main sheet to include it. It simply calls start layout main when player finished selection and all selections are store globally.