I don't have a CVT, but I have been changing tranny fluid in my own vehicles for over 20 years and have always used the manufacturer's fluid.
To answer your two generic questions, I would suggest using the correct Nissan fluid, which to my knowledge is only available at the Nissan dealerships.
Over the years, I have heard numerous stories before the age of internet forums and also on internet forums about cars having transmission problems because of the wrong tranny fluid. To be fair, I do not have first hand knowledge of such a problem, and stories no matter the source should be taken with a grain of salt.
My logic is, on a typical drain and fill you might use four quarts, a flush may use twice that much. Buying fluid off the shelf at a parts store might save you 2 - 4 bucks a quart over the manufacturer's fluid. So you save maybe $10 - $30. Consider the cost a transmission failure, it is probably somewhere around $2000 - $4000. The cost to risk ratio just doesn't make sense to me. Manufacturers select a fluid formulation based on the materials within the transmission and the performance specs of the fluid to work correctly. I don't have more knowledge than the Nissan Engineers that decided on the fluid formulation for that transmission.
You will hear other opinions that may be equally valid. Your choice.