What is "Sofer"?
Malbim (24): He writes all who come to the king for Mishpat.
Were they the only Kohanim?! And why does it mention above (8:17) Tzadok and Achimelech?
Radak: They were officers over the Kohanim. Even though Evyasar was deposed (from being Kohen Gadol), he was not totally deposed until Shlomo became king.
Malbim: Before, Evyasar was Kohen Gadol. After he was not answered via the Urim v'Tumim, he was demoted, and he was at the same level as Tzadok.
Above (8:18), it says that Bnei David were Kohanim. In Divrei ha'Yamim I, 18:17 it says that they were "ha'Rishonim l'Yad ha'Melech." Why are they omitted here?
Malbim: Above it means like it says in Divrei ha'Yamim 1 - they were first at the king's hand ['Rashi' there - to do all his needs]. After Avshalom's rebellion, David demoted them, for he intended to make Shlomo king after himself, and he feared lest another son rebel [and try to make himself king], if they are foremost in the kingdom. He appointed Ira ha'Ya'iri to rule over David's house and give counsel.
Malbim (Divrei ha'Yamim I, 18:17): Initially, they were only first at the king's hand. Later, they became Kohanim (great officers appointed over everything, or Chachamim). Similarly, above it does not say that Benayahu was Al ha'Kereisi veha'Pleisi; he was appointed over them only later.
Above (8:18), it says that Bnei David were Kohanim. In Divrei ha'Yamim I, 18:17 it says that they were "ha'Rishonim l'Yad ha'Melech." Why are they this omitted here?
Malbim: There it means like it says in Divrei ha'Yamim I, 18:17 - they were first at the king's hand ['Rashi' there - to do all his needs]. After Avshalom's rebellion, David demoted them, for he intended to make Shlomo king after himself, and he feared lest another son rebel [and try to make himself king], if they are foremost in the kingdom. He appointed Ira ha'Ya'iri to rule over David's house and give counsel.