I am trying to modify how Icons only taskbar plasmoid works when switching from an active application to another.

I want to modify 2 things:

1) When I click an icon of an application that is not focused, the action would send me to the last active window of the application (whether the application has one or more windows).

2) When the user right clicks on the icon, I want the context menu to display the list of windows of the application, a separator and then the current context menu.

For now, I have successfully achieved item 1. I couldn't get a list of the application windows in order to add them as menu items. I managed to modify the plasmoid context menu generator and I could add some menu items by hand with static text, but couldn't create a loop outside a context menu.

Could you give me some directions? Thanks in advance.

I'm not a dev so I don't have an exact answer, but perhaps until you get one, I may steer you in the approximate direction...

Have you looked at the regular Task Manager plasmoid? It does grouping of windows, by application name - perhaps the code used for grouping can give you the info you need for the list of open windows for an application.