I emailed the Pilgrim's Office last week and they have answered.
Their answer:
You must...
1. Walk 100 kms, which do not have to be continuous.
2. However, your last stage must be into Santiago. For some reason the person who answered considered Arzúa - Santiago to be the last stage. IMO, that's a long and not standard last stage.
3. Get two stamps per day, but still only on last 100 kms
That was it.
This answer fits the requirements as stated on the Pilgrim's Office's website. However, what remains unclear, at least for me, is what the Pilgrim Office considers as the last stage on each route. I am guessing that these would be Pedrouzo, Padrón, Sigüeiro and...
Happy New Year and Buenos Caminos in 2025