The dilution of Elven blood and subsequent mortality
 

Imrahil of Dol Amroth "had Elven blood in his veins" but was mortal. Does this mean that as the proportion of such blood is diminished in succeeding generations via marriage with mortals, that the bloodline reaches a point where death happens? And at what stage does that happen? 75% 50% 25% ? And why aren't some of Imrahil's ancestors still alive, if they had a high proportion of Elven blood?

In the same vein (no pun intended), Orcs were originally Elves who were tortured and corrupted by Melkor. Are they immortal, or has the same dilution happened to them? And if so, who did they interbreed with?

No, it doesn't work like that. Mortality is the default setting for human/ elf crossbreeds because Death is the Gift of Eru to men and may not be withheld. Tuor is a bit of an exception here but as a chosen tool of the Valar I suppose different rules apply. Mithrellas' children were mortal and presumably that was why she left because she could not bear to see her family age and die. It is a bit tricksy since Dior and Nimloth aren't properly accounted for, but the freedom of choice is a special grace granted to Earendil as a pawn of the Valar and extended to his sons.

Imrahil's family is longlived because they are pure Numenorean rather than part elvish. They are nevertheless ennobled by the elf influence.

I assume Mithrellas would have renounced immortality by her marriage to Imrazor, although, like Arwen, she would not have aged as quickly as them, so might indeed have wished to avoid seeing them die.

An elf cannot renounce their immortality anymore than a Man can decide to be immortal. It's quite impossible. There have only ever been two exceptions to his: Luthien and Tuor.

If the story of Mithrellas is true then would have returned to Aman at some point or wandered alone in Middle Earth forever separated from her husband and children.

As Mithwalen said mortality if the default setting for everyone with mortal blood, unless some other doom is given to them.

Oh I. Don't think she would have the right. There is no suggestion that Idril renounced her immortality on marrying Tuor nor Nimloth on marrying Dior who was born of two mortals whatever the bloodline. I appreciate he probably didn't have a hot lineto mandos but think of. Gwindor saying that marriage to Turin would leave Finduilas to a long widowhood. Arwen has the choice because of her heritage not her marriage imo.

In the primary story of Lúthien and Beren, Luthien is allowed as an absolute exception to divest herself of 'immonality' and become 'mortal' — but when Beren is slain by the Wolf-warden of the Gates of Hell, Lúthien obtains a brief respite in which they both return to Middle-earth 'alive' – though not mingling with other people : a kind of Orpheus-legend in reverse, but one of Pity not of
Inexorability.-Letter 153

Immortality and Mortality being the special gifts of God to the Eruhini (in whose conception and creation the Valar had no part at all) it must be assumed that no alteration of their fundamental kind could be effected by the Valar even inone case: the cases of Lúthien (and Túor) and the position of their descendants was a direct act ofGod.-Letter 153

Luthien and Tuor were direct acts from Eru and can not be expected to happen again.

the problem of the Half-elven becomes united in one line. The view is that the Half-elven have a power of (irrevocable) choice, which may be delayed but not permanently, which kin's fate they will share. Elros chose to be a King and 'longaevus' but mortal, so all his descendants are mortal, and of a specially noble race, but with dwindling longevity: so Aragorn (who, however, has a greater life-span than his contemporaries, double, though not the original Númenórean treble,that of Men). Elrond chose to be among the Elves. His children – with a renewed Elvish strain, since their mother was Celebrían dtr. of Galadriel – have to make their choices.-Letter 153

So far as I know only Elrond's line, including his parents and brother were allowed to choose a Doom. Anyone else is mortal. Imrahil's ancestors were Númenóreans/Dúnedain. The King [Aldarion; 6th King] of Númenor's wife, Erendis, called the Men of Númenor half-Elves, especially the Kings and their relatives, but they were Men.

As I remember, Mithrellas went missing after shacking up with what's-his-name (see bold underlined) of Numenor. It's not explained where, why or how, but left to speculation. I like to imagine that hubby was a pushy, bossy-boots Numenorean male who kind of imposed his will upon Mithrellas, and that she did a runner at some point, after she got jack of him.

There's been disagreement in discussions about whether or not the first kids of this union of Silvan and Numenorean were included in The Choice of the Peredhil. That's because Mandos's decree about The Half Elven originally cited only the line of Earendil as included by the choice of mortality/immortality. Though if you look at LotR appendices, the Choice of the Peredhil is not circumscribed in this fashion. I've always thought that had Mithrellas nabbed the kids, chucked them on a straight-road boat and sailed into Tol Eressea that they would have been permitted the choice. I also reckon that bossy daddy Numenorean meant that the kids were pushed into taking the mortal choice....