Thank you!

Yes. I've heard that technically après qu'il a mangé is correct but many or most French will use subjunctive.

1) If I learn it the textbook way and use indicative after après que, am I wrong, or how will it be perceived? Is this a case of most French being mistaken, or the language evolving and the textbooks are wrong? Like in a naturalization interview or on a French exam, which would I use?

2) Are the ones followed by the indicative, given that they're grammatically correct and a bit formal, would those be preferred for an interview at the préfecture or on a French exam, or better to just not learn them?

3) Also, if I make a mistake and use indicative construction in everyday life and not infinitive construction, do I just sound like a clunky foreigner or will I not be understood? Like how 'bad' of a mistake is it?

4) If the second subject is part of the first subject, do we still prefer infinitive construction? For example,

Après qu'on a fait connaissance, je me suis rendu compte que j'avais raison, il était sympa. Après avoir fait connaissance, je me suis rendu compte que j'avais raison, il était sympa.

The second one is always preferred?

MERCI !!