As
@Tincatinker and
@SYates have said, it doesn't really matter where you start, or what combination of routes you walk, so long as you do the final 100k along a recognized route. (And the recognized routes include a lot more than you've listed above. When you scan the QR code on your pilgrim's passport, you'll see the others).
Where it might be confusing, however, is that in registering for the Compostela, whether through the QR code on your passport or in person at the pilgrim's office, you have to list one camino or the other as the camino you followed. You can't list both, but it doesn't really matter, since the compostela itself does not reflect the route you walked, and all they care about is that you walked the last 100k along a recognized route.
This fall, for example, my wife and I followed the Camino Baztan from Bayonne, France, to Pamplona, then walked the Frances from Pamplona to Sarria. From there, we bussed down to Monforte de Lemos and walked the last 100k on the Invierno. In the pilgrim's office -- to the extent they looked at our passports at all -- they were apparently satisfied that we had walked (somewhere) for the requisite 100k and handed over our compostelas.
However, if, after having walked a combo camino, you also want the separate distance certificate, it gets a little funky (that is, if you want the distance certificate to reflect what you actually walked). That's because, if you fill in Invierno as the camino you walked, you're then asked for the place you started, which is limited to places along the Invierno. There's no option to then enter a place along the Frances, so your distance certificate will reflect only what you walked on the Invierno. If, by contrast, you enter Frances, you can only enter starting places along the Frances, and your distance certificate will then reflect the distance along the Frances between your Frances starting point and SDC, which will be somewhat shorter than what you actually walked on the Frances and Invierno together.
We ourselves were dazed and confused by the process when we tried to register for the compostela through the QR code, since there was no option for multiple caminos or for the Camino Baztan at all. When we presented this conundrum to the powers-that-be in the pilgrim's office, explaining where we had started and where we had been, they were unfazed. Since our starting point, Bayonne, was a recognized starting place for the Norte, they simply entered Norte as the camino we had walked, even though we hadn't spent an inch on it!
None of this mattered for getting our compostela, but it did get nutty thereafter. The woman who actually gave us our compostelas, apparently impressed by our two passports full of stamps, then urged the distance certificate on us. We were happy to make the requisite 3 euro donation, but when we left the office we found ourselves to be the proud possessors of distance certificates attesting to our 874 kilometer walk on the Camino del Norte!
Maybe -- if you want the distance certificate -- you'll have better luck in explaining what you did. But if you don't care, just pick either the Frances or the Invierno as your camino, and don't get too hung up on the details.
your post but the tale you tell makes me 
