Good questions. Here’s a brief answer and the MapPress documentation may have some additional info.
1. Only custom fields that have actually been used in a post are available in the dropdown. You may need to create a dummy post and publish it to get the field to appear. Also, some custom field plugins use internal IDs that don’t match the defined name (I’m not sure if Toolset does this).
2. The overwrite option is only for automatically-generated maps, i.e. if you edit a map that was generated, it’ll be overwritten if you publish the post again.
I hope that helps.