Are there are “half-elves” in Glorantha?
The answer is of course yes. We know adventurers fall in love with dryads, and those spirits of Fertility and Plant no doubt produce offspring, which humans would indelicately call “half-elves.” Given that the elves are also Man Rune entities, this is entirely mythically appropriate. This also means that there are no doubt half-elves that result when humans and elves mate, half-trolls when humans and dark trolls (let us not imagine what would happen if a human and a mistress race troll mated), and even half-dwarves. In fact we know this to be the case, for Lord Pavis was the son of a human and an aldryami, and Karandstand Half-Troll had human ancestry, and so on.
And if your character gets a high enough Elemental Rune, perhaps they are verging on that sort of thing all by themselves!
There might also be half-rivers, and even half-winds!
This of course is how the world was created.
Don’t believe this is common. Pavis is unique and his ancestry and family is intimately tied up with the mythology of the founding of Pavis. Common, no. But it happens. And Pavis is not the only case.
In Glorantha, these mixtures would usually be unusual one-offs. Pavis, Flintnail’s half dwarf son, Urrrrgh the Ugly, etc. I can only think of one group that managed to maintain a stable enough population to form their own identity.