Finally, my phone arrived yesterday and it's a beauty! :-)
Unfortunately, the N9 Repomirror installation did not work. (I have tried both versions.)
Then, in the end, I decided to flash my phone with the 339 firmware variant available in the first post. (With the windows flasher with windows 10) And luckily, everything went well!
Repomirror installed, development mode enabled! :-)

But I have another question:
I want to flash the open mode "fix" (Image_2.6.32.54-openmode_l2fix) because I want to install ubiboot and Maemo Leste later.

So do I need to re-flash my phone again with firmware and emmc right before doing it, like this?:
flasher -f -F firmware.bin -F emmc.bin -R ; flasher -a firmware.bin -k zImage_2.6.32.54-openmode_l2fix --flash-only=kernel -f -R

-or -

Can I just flash the open mode kernel anytime I want to, like this:
flasher -a firmware.bin -k zImage_2.6.32.54-openmode_l2fix --flash-only=kernel -f -R

I would not want to reflash everything again if it is not necessary...
Thanks!!