According to the dictionary, the word مُمتِع can mean: amusing, agreeable, entertaining, nice, pleasant, pleasurable, readable, rewarding, watchable, enjoyable. So, I don't think 'David is fun' necessarily means that he is funny, but rather that he is a fun person to be with.
Fun fact: In Indonesian bible, king David is called king Daud. There are other names that are different with the english translation as well such as John=Yohanes, Matthew=Matius, Job=Ayub, etc. (Indonesian got some loanword from Arabic because of the Arab trade, so maybe it got something to do with it)
Wow! These case endings are way too difficult! I never heard the term "diptotic" until you mentioned it, so I googled it to find out how to do the case endings properly. The articles I came across where short and not presented in a way that made it easy, or even possible, for a beginning Arabic student to grasp. Coupled with the fact that Duolingo does not have in-depth explanations of grammar and that the TTS system is not accurate, I don't know how we students are going to get a handle on it. What are your suggestions?
If you're serious about learning Arabic, I'd invest in getting a good text book or grammar. The best grammar book I know is by Karin Ryding, which is very thorough and will see you through until you're an expert. There are more user-friendly text books, like Mastering Arabic, but they will skip over the diptosis, case endings, etcetera, because it makes Arabic more complex, and you can learn quite a lot without worrying about them. So it largely depends on the kind of student you are - do you want a good, solid framework, which may be complex, or do you want to learn quite a bit of Arabic quickly without being too fussed about the nitty-gritty? If the former, Duolingo is quickly going to get really frustrating for you. I'd advise on getting a text book, and ideally a teacher. My favourite text book is Cambridge's Standard Arabic - an elementary-intermediate course by Schulz, Krahl, and Reuschel. Translated from German, it's the best and most complete Arabic text book I've ever found. Not for the faint of heart - it's dry and hard work - but it will tell you everything you need to know.
Well, "fun" generally means that you can have a good time with him, but "funny" is a bit more specific and suggests that he makes you laugh. If you look at ConchiCastillo's comment above, she explains the distinction. Admittedly, her list of dictionary translations includes "amusing", which is basically the same as "funny". But that is only one among many.