+1 for #4. However, on user-defined series I think merging is preferable (#1), since one can use index for sorting. So I would suggest that if you run into a name conflict that only involves series-type columns that the name is left as is.
For example, let's say I have a series (built-in column) John Doe and it has books 1-6 in it and another series (built-in) Jane Doe with books 1-3 & 5-7 in it. If I proceed to create a user-defined series column and place the value of Jane Doe in it for the John Doe then I would only want to see two series on the reader: John Doe (six books) and Jane Doe (seven books), with one book appearing in both series.
Does this approach make sense?