Since I wrote that comment I've experienced this first hand (I was on a business trip to Buenos Aires last week.) In real life it's actually pretty easy to get used to, once you know the "dj" or "zh" sound is a "ll" or "y". I was only there for a week, and I was using it myself by the time I left.
Maybe this is a conscious decision by duolingo to expose us to both pronunciations. It's noticable that they don't seem to use the castillian lisp pronunciation for "z" at all though... Along with the fact that they ignore vosotros, there seems to be a definite tendancy to promote the latin american version of spanish.... which is fair enough, as it's probably more useful for most people.
Yes, I was in Buenos Aires between july,15 and 25 too and I've noted that they use "vos" a lot. A thing that doesn't exist in duolingo. The pronunciation of "ll" with "J" sound is a Buenos Aires castellano accent. If you go to Cordoba, Mendoza, Patagonia, Colombia, Panama or Cuba. It has the sound of "CH".
With the Spanish sentence, they are using the verb llegar, right? But it is translated into English as 'comes' because some people do not say 'she arrives at your house', but I would. Anyway, the verb venir is used when someone comes to the place of the person who is doing the speaking. In the sentence, this does not happen that way. You could say 'she comes to my house today'. 'Ella viene a mi casa hoy' (you can use llegar too, but you can't use 'venir' in the given sentence.
This sentence has created a lot of discussion. For my own benefit (and perhaps others) I want to point out that here Duo uses llega correctly as "comes" when using to. Earlier they used arrive to which I think should be arrive in or arrive at. Secondly we have the preposition of movement 'the subject has movement toward the object' which is why we need 'a' it isn't a personal 'a'. I believe that 'a casa' = home and 'casa' = house. BUT based on this thinking I don't know how to say "she comes to your home"? Perhaps its all interchangable. I know 'hogar' is in some dictionaries but not all.