Nothing is stupid here.
When you create a custom column in a list do a full crawl and you will see a crawled property available with the name ows_ColumnName.
You can't map a column to a Managed Property. You always map Crawled Properties to Managed Properties. So you will have crawled properties from site columns and the YES you can map as many crawled properties to a single Managed Property and Vice Versa. It's Many to Many Mapping.
Updated Answer: There is always this scene running as we have "Title" Column in all the lists(Custom). So if you create you own column in two different lists with same name it's not new.
To Be Noted: It's the search engine and Indexer responsibility to give back you accurate results. In the above case with two same column names in different Lists you will still have ONE crawled property as ows_ColumnName and ows_Title for OOB column. But you will see the items from different lists only. Crawled properties are only a blueprint they are not attached to a Single List.
Mark My words.