Some New Matters have popped up since reading the thread.
I loved reading the Letters quote in the post about Tolkien's comments in an 'seeming' unifying of the divided lineage in Aragorn and Arwen, and in relation to the Choice of the Peredhil. Poor Dior. He does leap out as an anomaly, no matter which way you look at it.
Mandos's decree was one about Doom, and it annexed (seemingly) two Elven bloodlines (Earendil Elwing), based on wording of Canon, as presented in the Silmarillion, and as published.
Although the Decree is seemingly basic, it is a precedent with implications: some are temporal (Dior being born prior to it, yet the reincarnation headache), physical (Maia bloodline is present, and (jokingly - is Elwing then the reason Vingilot flies), metaphysical (Elven/Human Unions and the question of Fea) and Spiritual (Valar Worship ideas, and their implications).
In relation to questions of 'Doom' (and Dior's was tied up with the Silmaril. Presumably, being Elwing's forebear he is well within right of claim of the Choice of the Peredhil. Why didn't he get it? Was it because Mandos doesn't 'know' who and who is not living (yet he mans the forts after death - shunting souls this way and that). Presumably he nabbed Dior and put him somewhere. (Upstream I read about possibility of reincarnation).
I add in the presence of Maia blood, as a glaring reason to ponder Dior as having High Doom. Canon is silent, mostly, about Dior, but with such strong basis in Canon-onic (is that a word?) features about the whole area, I would posit that a simple emendation to Annals was highly likely about any of the several areas raised here, had Tolkien been permitted to publish the Silmarillion before he died.
Then, as has been pointed out that the presence of human blood, "however small in measure" is it "disrupts" immortality, or is it alters "Doom--of Kindreds" (i.e., that latter is a question about the Gatekeeper - Mandos).
So - there is the double-Doom interaction question (your 'contributions to major events + your bloodline Doom).
These are questions broader than Dior's fate and I have put them here and at the Half-Elven Foundation.