is the Di2 adapter just at the end of the seatpost? OK to cut off part of the seatpost if I am not needing this feature?
FSA still makes that post in a non Di2 version, just have to look around a bit more. So to try and answer your other question, as long as cutting down the post still allows for the minimum required insertion length, then yes, it can be cut off. Simply take note of where the stock FSA minimum insertion line is printed on the shaft and the insert is a tad shorter that. I had to cut the FSA K-Force Sb25 post even though I needed the Di2 insert, the post was too long when inserted into the seat tube of my wife's Scott Solace and I was able to reuse the insert once it was cleaned of the bonding residue without issue.