1)
May one do Mitzvos for the sake of reward, i.e. that Hashem will fulfill that David's descendants will always sit on his throne?
1.
Malbim: Normally, one should not do Mitzvos for the sake of reward. Here, I intend to enable Hashem to fulfill His word. Hashem wants David's throne to be for all generations! This is like "Lema'an Asher Yetzaveh Es Banav? v'Shamru Derech Hashem Lema'an Havi Hashem Al Avraham Es Asher Diber Alav" (Bereishis 18:19). Whenever Hashem promised for the good, it never was retracted (Brachos 7a), for His word brings the matter to reality. However, here the Tanai [that David's children guard the Bris] preceded the promise, so it could be retracted.
