What’s actually kinda interesting is that Esperanto is having a moment like this, while technically you are to use the pronouns Li and Sxi, for he and her, Duolingo has a lot of the use of Si, which is a singular they, and since a lot of esperanto’s modern speakers are duolingo users, a lot of folks are just using si.
I speak Esperanto for 14 years now. And no, "si" is not a singular "they". That's a self-referencing pronoun. And if that usage is used for genderless addressing a person then this is simply incorrect usage, because people don't know how actually the language works. It's used in sentences like "li lavis sin" vs. "Li lavis lin". The first one says "he washes himself" and the second says "he washes him", the first references the person who executes the action to reference and the second says that the action is done on a different person.
If it comes to Esperanto and genderless usage then there ĝi (it) or ri (they). The first one would be more in accordance with the fundament of the language and the second is a new pronoun which is around since at least the 70s.
No need to misuse si.
That sounds interesting. Esperanto has no noun-declinations, it's an agglutinating language, you don't bend words (= declination).
But what is barely resembling that what you mention is the two cases of the language, which is nominative and the so called "accusative". Which is adding -n to words to make them an object, depending on whether the verb of the sentence needs one or not. This case also is not just for objects, but also for directions, for measurements and time. That combination normally confuses the heck out of people.
Which is why there is also an in-joke in the Esperanto community "don't forget the accusative", because people forget it or apply it too often.