That is a GREAT question. Thanks for sending it. At this time, this issue is not directly addressed in any published rules. It is addressed, however, in the upcoming Foreign Language rules and I'll be glad to give you the BANA-approved recommendation here. This is only a recommendation because the rule isn't published yet and theoretically it could be changed by the time it does get published.
The braille you transcribe is considered correct at the time of transcription. If this rule is changed in some way by the time it's published, you (and everyone else!) will follow that changed new rule. Either way, you are correct.
Foreign languge braille issues are now commonplace in non-foreign language textbooks, like math, science and social studies. Glossary sections are typically translated to Spanish and there are these parent letters in Spanish as well, found in many textbooks of various subjects.
You are correct. Braille this parent letter entirely in Spanish with the Spanish braille symbols and rules, including the special symbols listing for the accented letters and the inverted question mark and exclamation point, if used in the text.
Braille the Nemeth text according to code as straight math, the way you normally would. Anything here that's not Nemeth, that you would do in English if it were English, do in Spanish. You may encounter overlapping symbols, that is Spanish symbols that also mean something in the Nemeth context and vice versa, Nemeth symbols also used in Spanish. Ignore this and just use the correct symbols in both situations. The reader will get the meaning by context. No TN.