E. Why Avraham waited so many years to circumcise despite having fulfilled all the Torah:

From Torah Or:[1]  The reason Avraham did not fulfill the command of Milah until he was actually told to do so by Hashem, is because he wanted to merit the higher form of Milah which comes from above and is much higher than can be accomplished on one’s own. This is the reason that the verse states that Avraham “became circumcised” and not that “he circumcised himself” as this level of Milah was the level of above to below.

From Likkutei Sichos:[2] A stringency may not override an obligation. The entire concept that our forefathers accepted upon themselves to guard Torah and Mitzvos even prior to it being given, is a mere stringency and matter of scrupulousness which was a result of their righteous behavior. However, under no circumstances was this an obligation. Accordingly, it’s clearly understood that these stringencies [of keeping the 613 commands] may only be kept and guarded if they do not contradict any direct commands or prohibitions that they are already obligated in, as certainly a mere stringency cannot override an obligation. This is one of the explanations behind why Avraham did not circumcise himself until he was directly commanded by G-d to do so, despite it being claimed that he kept all the Mitzvos prior to them being given. The reason for this is because it is an absolute prohibition under the seven Noahide laws for one to injure himself and spill his own blood. Hence, Avraham had to wait for a direct command from G-d to do so and override the general prohibition that he was obligated in under the seven Noahide laws, and could not choose to do so himself.

[1] Torah Or p. 13

[2] Likkutei Sichos Vol. 5 2nd Sicha Parshas Vayeitzei