Working under what I consider to be the safe assumption that you mean Armada rather than Amanda, I point out for clarification that the Armada took place after (and, I think it's safe to say, largely due to) the death of Mary and ascension of Elizabeth, who was religiously and politically much less friendly to Spain.

I am afraid I somewhat sidetracked the conversation to focus on the details of the Armada, so I will create a new reply to address a different aspect of the answer to your question.

Note that at the time Philip II and Mary I were married, Philip already had a son and heir, Carlos. His first wife, Maria Manuela, had died in childbirth. The expectation of the English nobility at the time, then, would have been that the most likely scenario would not be a personal union, as Carlos would inherit Spain (and Portugal), while the hypothetical child of Mary would inherit England. There would still have been a chance of a union if Carlos predeceased Philip or, I suppose, died without issue himself, while Mary's child survived with issue. In other words, a union was a possibility (or, depending on political stance, a threat) rather than a given; this, I think, speaks to the document that /u/freedmenspatrol provides with specific reference to the requirement

that the said lord prince shall not lead away the aforesaid most noble lady the queen , out of the borders of her Highness’ realm, unless she herself desire it, nor carry the children that shall be born of this matrimony out of the same realm of England, but, to the hope of succession to come, shall there suffer them to be nourished and brought up, unless it shall be otherwise thought good by the consent and agreement of the nobility of England: and in case that no children being left, the said most noble queen do die before him, the said lord prince shall not challenge any right at all in the said kingdom, but without any impediment shall permit the succession thereof to come unto them to whom it shall belong and appertain by the right and laws of the said realm.

For what it's worth, the reality turned out to be a complicated scenario in which Mary and Philip did not indeed have any children, but Carlos did in fact predecease his father; there could indeed have been a union of the kingdoms, as Philip's eventual heir, Philip III, would have been after a child of Mary in terms of age. As to what would actually have happened if Mary had given birth before she died and left her child in some sort of regency, this turns into the sort of what-if that, as you point out quite correctly, is unknowable. To the point of your question, then, the expectation would at least not have been that a union was the most likely outcome of the marriage producing issue.