Regarding item 2) I do nott really know as I never change these fieldsd from the main screen - only via the edit metadata dialog. One slight anomoly that might be relevant - Calibre allows trailing spaces in the Series field which is more obvious if done via the edit metadata dialog. I exploit this for the case where different authors have used their own names for a series and there is a clash. I give one of them the trailing space so that they get treated and sorted separately by Calibre.
Regarding item 3) I suspect this is something to do with the auto-completion feature which ignores case on that field.
You might want to try setting all occurrences of the series to something completely different, and then back again to the exact value you want. You can select them by using an expression like "series:discworld" (without the quotes) in the Search field. If you then select the edit metadata option you will get the Bulk Edit dialog which allows you to change them all at once. Once they have all been changed to another value, you can then change them back to the value you want with the correct case.