well, that guy could be Ruthenian archer - he does look dressed differently then the rest - and he has long hair
Also, do you see the last guy who has 2 spears for some reason - I would guess he is preparing to throw them
Also - the first still alive pagan is also fighting with two spears
Regarding arrowheads in castles - they again could be left by Ruthenian archers fighting on Lithuanian side, or archers firing from outside in.
And regarding Dlugosh - I am aware of his subjectiveness regarding Lithuanian rulers, but in the case of throwing spears detail - that has nothing to do with Jogaila or Vytautas.