[BetterRed]

If you set the Author sort name algorithm tweak to 'copy' and then go into Manage Authors and hit the Recalculate all author sort values button you'll get what you want - Mark Twain will be amongst the M's rather than the T's.

It may take a minute or so to take effect while the database Authors table is rewritten.

The default settings are the way they are because calibre is an e-book LIBRARY manager, not an ebook FILE manager. Bricks & mortar libraries and book stores shelve books by author last name (LN, FN), which is what the calibre default settings model.

This won't be the last feature of calibre that will need adjustment to its settings and customisations to meet your specific idiosyncrasies. So, if you aren't prepared to spend a couple of minutes changing a tweak and clicking a button to get authors sorted the way you want (FN LN) perhaps you should use another ebook library manager.

BR