When I researched CVTs for my Altima, I found some warranty claims were denied because the exact correct fluid wasn't used. I believe you don't need Nissan to do the job though. I just asked my mechanic yesterday if he could do the tranny service without voiding the 200000km factory warranty. No problem, they've done lots of mfr warranty stuff. They just need to keep documentation that was done properly. This is important to me cuz I am miles from a Nissan dealer, and from civilization altogether!
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.