oduino:
Buying just another Computer seems not a viable solution to me.
I did not suggest that. And you say you already have a Mega.
Load the Mega with the second example from Serial Input Basics with the function modified to receive from Serial1 and then you will have a simple intermediary between your working Arduino and your PC or RPi.
It is straightforward to acquire "root" privileges. On an Ubuntu system you prefix a command with sudo. There is sure to be an equivalent for the RPi.
But I would be slow to meddle with the Python system until you are sure that it is broken - so that there is ZERO possibility of making it worse 
...R