Of the many letter assignations I've seen, I never see the Mother letters put
where you put them. Often Shin is between Chokmah and Binah, Aleph between
Chesed and Gevurah, and Mem between Netzah and Hod.

Could you link to a Tree that shows them as you have described them?

Also, Da'at is considered to be a point of intersection and not a sefirah
itself.

I am interested to see which author you are following. There are so many
different Trees on the web, and the letters are put just about everywhere.

Also, please describe for me this term "Double letter sefirot".

Could be a lovely conversation. Not many seriously discussing it.