I don't know the answers to all your questions, but I'll try my best.
Main
The Raspberry Pi (3) is a full scale 64 bit micro computer with the 1.2 GHz processor, custom Linux distro based on Debian, on board Wifi, Bluetooth, HDMI out, as well as the ability to use GPIO and extension boards
The Arduino is simply a microcontroller motherboard with a max 48 MHz processor and can be programmed but can only run that one program at once.
Bonus 1
It seems like it's just different types of processors on the different models
Bonus 3
While most of my experience has come from playing around with the Pi, from what I was able to gather it seems that the Arduino is a better choice for IoT or DIY, depending on how advanced you want the actions to be
I don't know the answers to all your questions, but I'll try my best.
Main
Bonus 1
Bonus 3
Bonus 4