If you are not concerned about getting a compostela, then it is no problem! Dee isn't concerned about getting a compostela, either. Great! I was just responding to your question "Why wouldn't you qualify for the compostela?" And the answer is "If you hadn't walked all of the last 100 km (taking a boat probably doesn't count as walking)."
Where you start and how many stamps you get a day isn't the only thing. The other thing is how you move. If I started in Tui or Astorga or even Le Puy and took taxis every day for 20 km, then got a stamp in my credencial (two a day for the last five days), I could probably stay in albergues (especially, if I didn't walk directly out of the taxi and waited long enough to show up that it wasn't obvious I had taken one). I could probably get a compostela. I wouldn't have qualified for one. If you are walking the last 100 km, the people handing out the compostelas don't care how you moved before the last 100 km but they want you to have walked the last 100 km completely. (It's the same for riding a bike or a horse, but then they care about the last 200 km.)
Personally, I'm going to start in Porto and hopefully walk the whole thing (and at least the last 100 km) completely this trip. The compostela is nice to have. It is certainly not the most important thing. The experience is much, much more important. I didn't worry about collecting stamps or getting a compostela on my first camino in 1989. I enjoyed getting both on my second in 2016. I can picture taking the Espiritual Variante on a future camino. On that camino, I will be understanding if the folks in the Pilgrim Office in SdC tell me I haven't earned the compostela because I got a ride for some of the last 100 km.