Thanks for that, I figure out myself some of the things you say. (But I have some difficulties to reach the forum to edit my post).
Regarding synoboot, you say that I can take the bZimage and rd.gz from my fresh compiled replaced firmware, put them on a USB disk, mixed them with grub and it will make my synoboot ?
On real syno the synoboot seems different with grub efi, GUID partition and some specific code in it like cksum for example.
So the question is, we don't really care about syno are booting, we just need to fire our replaked kernel with some args on boot to do the trick ?