Hi Trundle,
You can also begin walking from Roncesvalles. Most of the Spaniards who walk the Camino begin there. I too, because I came north from Madrid (via Pamplona) decided to begin in Roncesvalles. It was recommended to me since I am not a "seasoned" hiker like you (and so many others) that perhaps it wasn't the best plan to begin by crossing the mountain. Since your time appears also to be a bit tight, beginning in Roncesvalles would cut one or maybe 2 days off your time. I walked Roncesvalles to Santiago in 30 days (no rest days) and was really covering ground at the end. It could easily (and perhaps should have been 31 days) - just an other thought to throw into your muddle.
By beginning in Roncesvalles you still begin in the Pyrenees, you just don't begin by crossing the highest peaks. If you take the 18h bus from Pamplona, you will arrive Roncesvalles around 19:30h in plenty of time to obtain your credencial, get your gear into the albergue and make the Pilgrim Mass/Blessing at 20h.
Buen Camino,