My goodness, an approximate answer to the questions you asked are way to complex for me to be able to explain them using Chinese...
Let's see... For you to have some idea of the differences and similarities of the main Latin-derived languages you can have a look at this picture and try to.overlap the map of western Europe on it:
So a handy rule is that neighbor countries have the most similarities (similarities in the way they evolves from Latin， you can still feel the influence of Latin even if you cannot fully understand another romance language)... It's very likely that Italian is the language more close to the original Latin from where all the other languages evolved from, but at the same time is one of the less spread romance languages lol.... Actually the whole western part of the world is influenced to a certain degree by Latin, for instance, even though English isn't a Romance Language the grammar of the language and some key aspects of the logic upon it's built has some much Latin influence that it feels as it's the same Spanish structure for me... Of course, this might be an opinion based on my particular experience since I don't have a deep knowledge of linguistics.
My advice for you would be to learn the language you like the most if you're 100% for the fun or learn the one you can use to connect with the most people if you want it to have some utility... In which case it's defined Spanish (although I'm a little bit biased towards Spanish because in my opinion is both useful and fun to learn because we latin americans are crazy and most of us speak Spanish... You can tell the latin american influence of our ever-party culture is so big that most of the southern part of the US you can use Spanish as well as English in your daily live because Spanish is so popular between youngsters... I think that mostly is because of the music 😜
As for the Christian Church, I know for certain that in Italy they use Italian and Latin... But I have no clue what could be the one you are listening to, it depends on each church... Maybe you can give me a link to a recording of that and I can point you which one it is.
As for learning online, of course you need interaction.... But I think that you can just let it slowly get to you... If you're not learning it because of work or academic interest, you can take your time and let it sink into your brain... The key here is surround yourself with as much as the targeted language you are trying to learn as possible... It's as if you are a little baby again, and have to Learn everything from zero... Of course it's really slow this way but it also is the more strong and genuine way to learn a language and as far as I know you're not in a hurry 😂
Fair enough.