Thank you for this clarification.
In your original post you posed this question "
I am wondering if this is a good alternate route?"
Various people have since asked you for your reasons for wanting to walk this route. Based on your original post I had assumed that you wanted to find an alternative route to Roncesvalles that didn't involve walking over the top of the Napoleon route.
I now understand that my assumption is incomplete and that what you want to do is to walk up the Napoleon route as far as Orisson and from there to Roncesvalles without walking over the Napoleon route pass.
There are two obvious ways of doing this and many other non obvious ways. The two obvious ways are to walk further up the Napoleon route until you reach D128, then walk down to Valcarlos and then back up to Roncesvalles. The other obvious way is to turn around after reaching Orisson and walk back down the same route to SJPdP and then from there to Roncesvalles via Valcarlos.
Of these two options, you have already seem to have identified the "best" and if you are seeking validation of that from this forum that D128 is the best of these two then I am happy to tell you that I agree with you.
As
@Tincatinker has already said, it is highly unlikely that anyone on this forum has ever walked this route for the reasons that
@Kathar1na has so eloquently illustrated and so I suspect that you won't get any better advice.
As for answering your original question about D128 being a "good alternative" I think that the vast majority of Pilgrims have voted with their feet and are saying that they don't think that it is a good alternative. It is a big world though and it is possible that somewhere there is someone else that may agree that D128 represents a good alternative way to get from Orisson to Roncesvalles.
I wish you a pleasant walk when you walk this route and invite you to report back with your opinion on its value as an alternative route. Kiaora.