/u/needsteeth

I think it will be great for your historian friend to provide references ;-)

Aspects of this question has been discussed here with some really good comments by /u/Tiako and /u/Itsallrightwithme.

But further on context. All empires of that era were very decentralized. As a result, officials at all levels had (and still do have ...) tendency to exaggerate. Especially given the long lines of communication. So the call to "let's invade China!" by the governor of the Philippines has to be read in this context. That thread already spoke of the infeasibility of that undertaking. But we can go further look at Spain's strategic interest in the region.

There were several competing European powers in the region: Portugal and Spain, the Dutch, and later on the English. Portugal and Spain were united for some time, from 1580-1640. During this time there were extant collaboration as they faced a common enemy, the Dutch. But given the system of governance, trade and maritime security was run separately. This meant Portuguese merchants took the eastern route (round Africa) and the Castilians took the westerly route (first to the Americas, overland to the west coast, then across the Pacific).

There was significant trade in the South China Sea region, between ports of China, Malacca, Batavia, Manila, Nagasaki. Japan and Spanish Americas were producers of silver, that tended to be desirable in China. Depending on the exact time period, there were trade restrictions of certain routes and access to certain ports. During such time that Japan could not trade with China directly, some trade went through Manila instead.

So depending on the era, the strategic situation would have been different.