But . . . if you can grow it from seed, what would you get? AFIK, hybrids
(almost by definition) are mules (sterile) and can only be propagated
vegetatively.

That is not at all true. It is more likely to be true in animals, but people
have been breeding plant hybrids for hundreds of years. God has been
experimenting with them for longer than that.

If seeds are viable, that says sexual reproduction works, and the resulting
seedling
could/would/may be entirely different from either of its parents

Not necessarily. First-generation plant hybrids are generally intermediate
between the parents, with occasional exceptions.

(and how could you be certain who both parents were, anyway, plants and
their pollen being the promiscuous things that they are?)

Not as promiscuous as you think. When a bee sets out for a certain plant, she
will go from one plant of that species to the next until she is through.
Pollinators rarely skip around, which is why natural hybrids are actually quite
rare. Unusual hybrids may occur when man plants near each other two related
plants which come from opposite ends of the earth & have no natural
hybridization barriers. This is how we got the Dunkeld larch, and was probably
the origin of the Pyracomeles and xFatshedera lizei. The parentage of such
plants is usually obvious to the naked eye, & also determined by logic due to
the presence of both parents, but today many natural hybrids are being
determined by advanced microscopic techniques.

Presumably (?) pollen from either of the originating species could have done
the dirty deed.

The name of a plant hybrid is the same regardless of which is the seed or
pollen parent. I am not familiar with all animal hybrids, but the differences
between hinnies & mules, and ligers & tiglons, is significant enough to warrant
different names.

If it DOES reproduce sexually (and more or less true to type) wouldn't that
be the first (or even last) step to species-hood?

It might in nature, but here we are talking about a spontaneous garden hybrid,
which is something different.


Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)