Not quite sure what you mean by external and internal button, but here's a photo of the new "v3" eR9 shifter from a set that I just got. Apologies for the cling film.

Is there a 2025 v3 version also on the er9 ?

Urgh, mechanical will always be challenging with these frames that are meant to be used with electronic. I would still try to have 2 lines on each side of the stearer - similar to my picture but just 2 lines on each side instead of one each. That way you should have a bit more space for each line. Have not installed mechanical on this, so no hands-on experience, just throwing out some ideas. I think Serge hast some experience with it.

Thank you for your help ! I will give it a try

How do you manage cable routing near the steering tube ? I put all cables on the right side and it kinked my front brake hose . Putting 2 cables each side seems impossible.

Hello guys !
My shimano 105 mechanical front derailleur comes with a cable stopper . Should I run the hose all the way to derailleur and remove the frame cable stopper ?

Thank you !