Yes it's a great feeling, very satisfying. I struggled with things like conjugations/objects/clitic pronouns and although I'm not perfect yet, I have a fair idea of what's going on!
Duolingo might say they teach language by immersion, I don't really believe that. But their complete lack of guidance means at least when you go out to learn the grammatical rules yourself, the effort leads to a deeper understanding!
"How can you" is present tense, while "How could you" is present perfect (I think; it's another tense either way).
I'm not sure what's different about "How were you able?". It might be an alternative, but I (as a native speaker) would use "could" long before I used "were able" in normal English.
I think it wouldn't sound good "cosa ha", because the word "cosa" ends with the same vowel "a" that the word "ha" starts( in the spoken language, because "h" is not pronounced while speaking in Italian). While "come" on the other hand, ends with a different vowel ("e"), so we can't apply the same rule here.
I'm not a professional, I just wanted to help a little with what I know.
Have a nice day :)