1. with a single Home key press to go to the start of display line (as in many other editors) instead of a start of real line

deselect `menu->Edit->Preferences->Editor->Features->"Smart" home key`

2. with a single Shift+Home combination to select to the start of display line (as in many other editors), instead of.. How am I supposed to select to the start of display line, if Alt+Shift enables the column-editing mode?

Try after de-selecting the above option.

3. with the subsequent press of (Shift +) Home key to go to (select) the start of text on that line, and with the subsequent presses of Home key to alternate between start of line and start of text (smart Home key functionality)

Indeed what smart home key does if the option above is selected, except it goes to the start of the text before the start of the line.

Basically what you want is for the current "smart" home key to go to start of line before start of text, but, otherwise be as it is.

Well, if someone wants to make that another option it should be ok, but not as a replacement of the existing functionality.

go to the start of display line

As was linked on #389 Alt+Home and Alt+Shift+Home are provided directly by the Scintilla editing widget, they are not handled by Geany which does not know about the display line.

So changes to their functions need to be addressed there or the keybinding in Scintilla could be unset at startup and the keybindings moved to Geany where they can be re-bound, but the functionality would still be what Scintilla does.

Pull requests to do that and providing options to add alternative functionality that do not remove current default functionality are welcome.