Translation:This movie was very interesting.
Indeed! Movie is old fashioned in the UK, short for "moving picture". Film is the term in the UK.
おもしろい is an 'i' or true adjective and it's ending changes to show tense and positive and negative - sort of like having です built in? na adjectives however like しずか or きれい need help to modify nouns ie. they can't modify nouns directly, な helps them to modify nouns. The endings of na adjectives don't change so です does that for them.
Because 'Omoshiroi' is an 'I-adjective' it wants "Katta" instead of "Deshita"
おもしろい can mean both interesting and/or funny. But funny as is in funny haha - a funny joke etc. The difference is that たのしい means fun as in enjoyable. おかしい (not to be confused with おかし which means candy/lollies) means funny as well BUT it means hilariously funny or funny in a strange way and it can also mean strange - as in funny in the head. My understanding is that おもしろい means interesting or funny - amusing - like chuckleworthy, whereas おかしい is tears streaming down your face funny.
I guess this is a question about language in general but why would one refer to something like a movie in the past tense? Would it count as present while it's still in treaters or if the people in question are currently watching said movie?
The speaker is referring to the movie in the past because they're clearly talking about something that has happened in the past ie. the speaker watched the movie and that is why they can now say "it was interesting/funny".
Duolingo could be better. If no updates like this are made then it makes "learning" more confusing than engaging. Same goes with the leaderboards where clearly now, not the quality of your learning but your score only matters for any kind of lesson and language you take. For instance, if one is a proficient English speaker and in the lead on the scoreboard in English, while another has a lower rank, a proficient Spsnish speaker and learning French, what exactly does this mean?