How stupid is this PC-race rejiggering?

[quindraco]

The surface of the same island chain hosts orcs. Orcs are all biologically female and reproduce via parthenogenesis.

You lose me here. Just to clarify, are you saying the orcs are clearly female and are using parthenogenesis as a fall-back (i.e. it's a setting mystery where the males are), or are clearly parthenogenic, and non-orcs are idiots who refer to orcs as female based on a shallow understanding of their body structure? I'm asking if orcs are chickens and the absence of roosters is a setting mystery, or if orcs are amoebas and only a moron thinks of them as having genders.

Gnomes consider it a high honor to distinguish themselves enough to allow such a blending of characteristics.

If they're sophonts, you need a lot more work to build a culture built around a death cult where the highest honor involves killing yourself. If your intent is that this works like the Star Trek Trill, with Gnomes as the Trill symbionts and Orcs as the Trill, and the result of the meld is a new entity consisting of all member entities "at once", you need world-building on par with, well, the Trill.

Adventurers (and notable NPCs) may come from two strands:

1) An orc/gnome rejects the standard cultural beliefs and seeks to prove the self is as vital as the blending, setting out to achieve something beyond the islands.
2) A dire event throws a hero of the united people into the broader world.

Wouldn't the latter be radically more powerful than the former, assuming your meld mechanics exist and are the upgrade you've protrayed them as?