@theducks - Yeah my 9 out of 10 times for using this plugin is to rename a series - for the reasons you have stated since calibre tends to muck things up. I have a similar "complaint" when you rename a series in the single Edit Metadata dialog as it is the same problem. The use case being your other books in the series are called "Foo", you do a metadata download and it gives you the right series index but calls it "The Foo". While in that dialog you rename "The Foo" to "Foo", and calibre decides that is an excuse to then go and overwrite the series index to make it whatever number would be last+1 in the series, rather than the value you had/just downloaded. Rather annoying.
Anyways - back to things I can control.
. I know you are a big user of this plugin so I was interested whether there was anything in particular you find yourself repetitively doing that could save you a few steps. Would the changes to the lock dialog I suggested above be useful?
I would definitely keep the old series column as is. It was just the new series column I was thinking could be made into an editable numeric only one.
Your comment about "1 time fractional" - are you talking about the "Start at" number at the top? As I looked into that not so long ago and there was a world of pain associated with that. The two main problems from memory were that (1) the plugin will default this textbox to the current lowest in the books you have selected, and (2) how does the plugin try to guess what your increment is? Whether a number is fractional or not is used by the plugin to default a hierarchy that it can preserve, by working on the assumption that anything fractional before you opened the dialog up should stay fractional by default. Since a de facto convention in calibre has been to put short stories as .x in a series, and I would suggest is the largest use case. Trying to then imply that fractional has "other" meanings like the 1.23 omnibus etc just makes it all fall apart, and means users just wanting to rename a series will have their numbering potentially screwed up.
I understand that if you were managing some magazines, and wanted to use 2012.01, 2012.02 etc then being able to specify 2012.01 as "Start at" could be handy. But then 2012.10 for a date based series vs 3.10 for a short story in a conventional series have totally different meanings when it comes to setting increments/preserving indenting. All just way too hard
As I posted once before you can work around all this for this particular case using the assign series index function or locking at original values and unlocking those you want to change. It is a couple more clicks but I can't see an easier way around it...