I think technically O Porriño is just over 100 km from Santiago (Brierley, for example, has it at 103.2). So if one were to skip the Tui to O Porriño section but still walk the rest, one would still qualify for the Compostela. If the section after O Porriño is skipped, the same cannot be said.
Be VERY careful about how tightly you try to shave the distance. The mileage tables in the Pilgrim Office computer are based on GPS data measured from the stairs of each town, village, or city’s main church or Cathedral. They are not subjective, and they are authoritative.
In places with no church, the distance from every municipal Albergue is recorded in the tables. They do check sellos if there is a question.
Thus, the Cathedral sello from Tui indicates you walked that distance, from there. But a sello from anywhere in Porrino will be credited as being from the church in the center of town, according to the standard table. This is 99 km NOT 100 km.
The Pilgrrim Office staff do not appreciate or negotiate with “Camino lawyers.” Some would disagree with this stance. But maintaining standards is critical to them.
The distance tables are what they are. I always recommend starting at the church, obtaining a sello there, or someplace immediately adjacent if the church does not have a sello.
So, the risk is that you could start from the edge of a city or town to walk the least possible distance. However in doing so, you may have cut it too closely. Often there are several Km from the church or Cathedral and the edge of town.
Again, be careful...caveat emptor...