You cannot replace the default, but you can replace the widget. That’s what the “Default List Item Widget” is for. Since you have more than one, I’d recommend creating a custom widget under Developer Tools and then you can choose that custom widget when you configure the Default List Item Widget.
Not by default. There is a one-Item-one-widget by default. However, you can reference and represent as many Items as you want in a single widget. For example the following widget shows the current temperature, has a slider to change the thermostat setpoint, and a button that can change the HVAC mode between heating, cooling, and off.
And a quick search shows someone has already published a standalone widget for this: Custom rollershutter widget with preset configuration. You might be able to take inspiration from that.
