Here in Portugal we no longer use the distinction legitimate/illegitimate children.
We say children born in/out of wedlock.
If Maria was to marry some bloke, that would not make him the "legitimate" father of Carlos and would not allow him to inherit the title.

Manuel could provide some answer if Maria's husband would adopt Carlos.
The couple would be Duchess and Duke Consorte, but even so, I guess that would not make Carlos eligible to succeed.