1. Catalan is a language, not a dialect. 2. All of Spanish America speaks Castilian. 3. There are very many literate Mexicans, including winners of the Nobel Prize in literature, and Mexicans are completely comprehensible to other Spanish speakers. 4. I, as a person who speaks educated Spanish and am really competent in it, can (a) INSTANTLY switch to using the vosotros form for you informal plural, it is no stretch, and (b) INSTANTLY switch to voseo for you informal singular, it is no stretch at all. 5. Forsooth, those kinds of grammatical differences among regions, and differences in regional vocabulary, are ELEMENTARY. 6. There are ways in which my Spanish is not as sophisticated as that of some educated native speakers but these subtleties lie at FAR more advanced levels than being able to understand regional differences in normal speech. 7. In academia, we hire on expertise and publications, not on accent. So YES, it is possible to be an expert on Spain without being a Spaniard or speaking like one, and YES you would still be respected as an expert.
I guess you can tell I had an argument with someone today.