Most of today's cartoon is canon (from History of Middle-earth volume X) but the bit about the souls of other Elves arriving in Mandos before Míriel is my own interpretation. (I know we've talked about this before.) As presented by Tolkien, it was't until Míriel died that the Valar were really confronted by the idea of the death of a souled creature, and it disturbed and confused them. This also leads to the implication, which I'll cover next time, that Death wasn't originally in the Great Music - or at least the Valar didn't notice it there.
I'm assuming that Námo has his reputation as a Man of Mystery to uphold, which is why he kept quiet about the other Elven souls appearing. But the Valar knew Míriel personally - I imagine she and Vairë swapped knitting tips - so they were aware of it when she died.
Part 120: That sleep of death
Next time: Part 121: What dreams may come?
Chibis by tektek.org