Another thing: what does RAR stand for and what are IASA frames?
To answer your first question, you theoretically can do 2+ Bairs in a row if you're up at a high elevation, or if you do a rising full hop Bair then rising double jump Bair, etc. The only stipulation being that Neon's double Bair combo cannot be performed in 3.5 as seen in the video
RAR stands for reverse aerial rush, which is running forward then tapping back and jumping. This will make you face backwards with forward momentum. Generally RAR means running forward, tapping back & jumping, then performing an aerial right after. In the example with Neon, he did not use an aerial until he was in range to hit M2K's Mew2, then performed another Bair immediately after without fast falling either of the 2 Bairs.
IASA means "instantly as soon as," or in layman's terms, the point at which you can cancel the animation of a move with any other action. For some moves this is very useful, such as Marth's Dtilt being able to use another Dtilt faster, or canceling the ending frames of the animation by dashing, shielding, jumping, etc.
To put this into context, I was stating that I was able to identify the first few frames when I could act out of Bair and immediately perform another action. In 3.5, the end lag is still funky and I'm still getting used to it >_>