First, Legolas & Gimli aren't at the Council because they "begged leave" to go
and visit Merry and Pippin. They probably would be included otherwise,
since Aragorn tended to be very inclusive. In other words, they
themselves opted to be elsewhere.
Second, I agree that Imrahil's elven ancestry is real, but wasn't "publicized"
because of the relative obscurity of the original couple, in contrast to the
more famous Noldor couplings. I wonder that Legolas didn't spot it in
Boromir, with whom he spent a lot of time, since Boromir's mother was Imrahil's
sister. I guess the answer is that, just as we hear of individuals in
whom their Numenorian blood "ran true" the elven ancestry is more obvious in
Imrahil.
Finally, I agree with the suggestion that a major reason Aragorn isn't pressing
his claim to Kingship at this point is that he is about to lead a "suicide"
mission and doesn't want other participants to feel commanded to
participate.
