Memorizing Randall is completely applicable in realistic play. People who know how to time him do it all the time. I think Toasty put it best when he said that "you can adjust your recovery and your edgeguards appropriately."
I understand that stating this makes it seem like you could just do the same with the 16 stage acid levels, but like you said, having to recognize 16 different levels on the fly and then determining what level is currently being represented is a far too distracting task to execute in real time. with Randall, you just associate numbers with locations and it just takes a brief glance at the clock. he follows a set pattern no matter what. Learning how to time Randall is generally not terribly useful because you can react to him anyway. attempting to land on Randall usually just causes the edgeguarding player to shift their edgeguard to accommodate him, but not in a radical way that could kill the edgeguarder.
So, the lava is in fact is random at the start of the match. Then you have to perform an impossible to process mid-match forced recognition of the stage's 16 different levels while focusing on fighting your opponent, dealing with the hitbox extending properties of the fleshy parts, and the unusual geography of the stage. Even if you are used to these things, they still come into play. The acid (that does lava damage) can also easily combo you to kill percents, gimp your recovery, and kill you if you misinterpret which of the 16 stages it is on.
why is this stage legal again?
it's 5 am at mlg dallas and i can't articulate. i hope i got my point across.