Considering what we would get in transfer fee for him i would keep Endo unless someone goes a bit mad and offers about what we paid .
Morton has the advantage of being HG and CG so has added "value" to us
I'd keep Endo, for all the reasons mentioned here.^As you say, no point in selling Endo. His value is relatively low, his wages aren't high and he's proved he's useful to come off the bench when games need closing out. Not to mention he is committed and has great work rate and genuinely seems to enjoy playing and wants to play for the club.
Only reason to sell would be if we need to free up foreign player numbers in the squad.
It's a squad game, and he's pretty handy at shutting up shop for the last 15 or 20 minutes. No frills, but a solid back-up.