'Foxfire Book' ordered :3 Never heard of it, searched and found it amazing. I love internet, thanks for sharing :D

Erkka, I see you are having a busy week. If we were neighbours I would gladly help, but since there is a lot of distance I just wish you all the best, you are doing a great job. Your idea was practical and very good.

My father was carpenter. I remember that there was a technique called "mão de amigo" (in portuguese, in english I don't know), used in that cases, to repair main logs of roofs. Here some photos I found on google:
http://2.bp.blogspot.com/-IfjaFE-qEL0/U5sOWXHOwOI/AAAAAAAAAos/3Us2foDGpY...

http://2.bp.blogspot.com/_wyaH2yzwhZ4/SJTcvvvNtKI/AAAAAAAAAAc/a1N4dYwyUW...

http://4.bp.blogspot.com/-wMJjvcGuXJw/U78SM5Wv3ZI/AAAAAAAAA4I/ZfaUzMiLVF...

This site has more photos of a repair work:
http://www.ct.ceci-br.org/ceci/publicacoes/96/656-boas-praticas-da-conse...

And here it is a video of someone doing it:
https://www.youtube.com/watch?v=rRdBROSw-tc

But it may require some experience, it is not easy to do it at first try. Like other people have said, if the main log is just rotten in the corner, it is easier, you could just lift the roof and insert a log as a pillar under the good part of the damaged one, or something like that.
These are only vague ideas, hopping to give some inspiration.

I admire you, from Philosophy Studies to Videogame Programming, to construction, agriculture, mushrooms and plants knowledge, music, etc.. You are a Finnish version of Leonardo da Vinci :D
All the best for this reparation project.