Welcome to the forum!
There are two routes from SJPP to Roncesvalles. Both are waymarked:
- One goes over the Napoleon route pass Orisson and Borda, which are the only two accommodations. There is no village. This is perhaps the route that has been most popularized recently.
- The other route goes through the village of Valcarlos, providing a slightly different but lovely experience. It just doesn't seem to be as famous in the pop culture.
You have accommodation in Valcarlos, so it makes total sense to walk directly from SJPP to Valcarlos. (Is this what you mean by the "alternative road route from SJPP"? It is not all on road, but others will be able to describe it better than I.) It doesn't make sense to walk to Orisson and then transport to Valcarlos. You would need to go through SJPP again.
If your heart is set on doing the Napoleon route (#1) above, you could walk to Orisson one day, get transport back to SJPP, and then get transport up to Orisson the next morning so you can continue to Roncesvalles.