In one of the threads of the above-mentioned discussion -- supposedly a native speaker -- writes that 'avercela' is always used when one wants to express 'I possess' instead of merely using 'avere' and saying 'I have'.

That helped me make better sense of 'avercela'. Would be grateful for a further confirmation by another native speaker.

I just gave "L'ho già" as my response and it was accepted with "Ce l'ho già" given as an alternative correct answer.

Ce non e necessario.

I hope you reported it because the use of the contracted form is not compulsory.

same question here

also question given no context why feminine "it" or "la" is not accepted

I don't think so, but if you see one like that, you should probably report it.

seeing as this comment was left almost over a year ago, is there anything that you've learned that you can use to explain the sentence structure to me? because i'm a bit stuck at the moment :)

I once heard a theory that over time, people speaking a language tend to simplify and simplify the way they speak to the point that they become difficult to understand. Even the best listeners don't catch every word that is said -- thinking about something else, etc. Imagine l'ho being said in a noisy room of chattering people with motorcycle noise from the street outside. So then people speaking these simplified languages start introducing redundancy to make things clearer, so that it isn't absolutely necessary to her every word or syllable clearly. "Ce l'ho" or "ce l'ho gia" has this kind of redundancy. (Sorry, Duolingo no accent on my keyboard)

Ancora=yet