The AkallabĂȘth tells us on the subject of Isildur's theft,
Isildur passed through the guards and took from the Tree a fruit that hung upon it, and turned to go. But the guard was aroused, and he was assailed, and fought his way out, receiving many wounds; and he escaped, and because he was disguised it was not discovered who had laid hands on the Tree.
This story may be slightly AU in that Isildur is not assailed directly in this inner courtyard. If I was being ornery, I'd say it could be argued that Isildur escaped the inner courtyard of the Tree and later roused the guard before he got out. It always seemed improbable to me that Isildur could escape all the way out of the palace, let alone Armenelos, without lots of help.
The idea that Morgoth would drink Isildur's blood is derived from this passage, also from the Silmarillion:
Thereafter the fire and smoke went up without ceasing; for the power of Sauron daily increased, and in that temple, with spilling of blood and torment and great wickedness, men made sacrifice to Melkor that he should release them from death.
I don't think MĂriel actually believes that Melkor benefits from the sacrifices; it's just sort of a colloquial way of saying "he'll be sacrificed."
The Elendilmir is a relic of the Faithful of great worth. Like the silver stars that the DĂșnedain rangers wore, but of higher lineage. For more on it check out the HASA resources entry. Very interesting stuff.
And finally, at least two lines are adapted from the miniseries "Into the West"; specifically, to dream whatever dreams she could, so long as when she woke, she remembered that PharazĂŽn and not MĂriel ruled NĂșmenor. and 'tis better to know an Orc and confound his purposes, than to leave him to his own devices.