Also, remember not to intellectualize things too much. The last 100 km means just that. As stated well above, you must walk the final 100 km on any Camino route terminating in Santiago to qualify for a Compostela. On a bicycle it is the final 200 km (from Ponferrada generally)
For example, you might have walked 690 Km to Sarria from St. Jean Pied de Port before bailing at Sarria and taking the bus into Santiago. This does not qualify for the Compostela. Only the FINAL 100 km is considered. Those are the rules. This happened to me for medical reasons in 2016 coming up from Madrid, but I was aware of it and it did not bother me.
Before that, you can do pretty much whatever you wish. In regards to obtaining the Compostela, the Pilgrim Office focuses on the final 100 km very closely. That is why you are required to have at least TWO SELLOs each day for the final 100 km. Each sello establishes a data dot on a continuous line leading to Santiago.
The Pilgrim Office staff are familiar with every stamp in every albergue, hotel and cafe along these routes. They will call out and even deny a Compostela to a pilgrim who appears to be skirting the rules or outright cheating. Yes, you would be surprised at the number of pilgrims who show up with a handful of sellos in their Credencials del Peregrino, demanding a Compostela.
Sellos are easily available just about everywhere on the final 100 Km. So, plan ahead.
Separately, if you wanted the optional (€3 donation) Certificado del Distancia (Distance Certificate) that shows when and where you started and finished, you would still have to prove the distance covered with appropriate sellos. Sporadic taxi hops or occasionally skipping a town on a bus are seldom an issue. But wholesale skipping over a lot of territory to report that you "finished" the entire 799 km
Camino Frances in 14 days, WILL produce a more exhaustive line of questioning and likely a refusal to issue the document.
Just sayin...
Hope this helps.