...and if pigs had wings, they could fly. What if the cross-breeding was intended to produce a near-microscopic dog and a horse size Great Dane, and the program was kept up for 10,000 years?
Undoubtedly Yes -- which suggests (to me) that they're still the same species.
This is not in the least "undoubted". If insisting on filling the fossil gaps is sauce for the goose, it is sauce for the gander. There were once Marsupial wolves in Australia--do you think they could have been mated with timberwolves because they were about the same size?
Do you think horses, zebras and Donkeys are the same species because they can produce offspring when they mate? If you think they are really one species, how do you think they are going to be merging back together, since their mutual offspring are sterile?
Seriously -- if x-great-grandchildren can produce viable offspring, why does that not suggest "same-species" to you?
OTOH, I do see your larger point, which is that eventually the two breeding lines should become different species, such that x-great-grandchildren don't produce viable offspring.
This is not in the least "undoubted".
I was referring specifically to the possibility of mating a modern-day teacup poodles, and the crossbreed great-grandchild of a great dane. I hold that that is indeed "undoubted."
As for your other examples, I'm not going to address them, except to point out that those near-relative species do in fact exist, and can mate to produce live, if not fertile, offspring. Obviously they got that way somehow. Beyond that I will say no more, except to point you to my previous comments on this thread.