Reserving Mega Lopunny + Clefable. Because I'm a mean person ;)
Actually, volcarona forces ferrothorn to stay in because if you switch and volcarona quivers, you lose. Meanwhile if you stay in and volc fire blasts you can still go lopunny and revenge kill. Also the fact that nothing can switch into volcarona in the first place except clef, but whats clef doing against a volcarona?Volc force ferro to switch so thing one thing it can set up on as no one is staying in with ferro against volc . Also most people would predict Lum so that would means that they may not thunder wave with starmie and go for scald in hope that they can kill volc with one or two rounds of fake out with Lop as that would be there but chance if rocks were up since lum get the kills of t-tar and lando-t if rocks are up .And also if landot and t-tar or bellow like 80% it force them out too.
choosing between Lum and LO if basically choosing weather you want limited set up opportunity but when you do set having the instant in or being forced to keep up rocks for the sweep but with easy set up.
Also Volc doesn't have to set up since it LO it can also function has a wall breaker as it is LO and does insane damage and out speed 4/6 mons on the team also if the opponent sack starmie to thunder wave volc like then azu instantly wins as starmie scalding azu to burn or paralysing it with thunder wave was the only way of preventing the sweep well lop can fake out but most the team would be dead by the time fake out kills .I think you have a too narrow minded approach to what volc can do to the team it doesn't have to set up.
but hey a comment like that was probably due since my explanation was lacking to say the least. If id explained all this in the original post this wouldn't have to happen my bad. But i do think to ability to set up on 2 pokemon is good enough them being clefable and ferro unless they make an ungodly predicting and thunder wave the volc but if I had not revealed my item yet that would be more than unlikely.
Again sorry for the lack of an explanation and I hope this answer you curiosity/question and over all it less volc can set up on a lot and more if volc set up the game literally ends as I can roost when lop switch out after faking out. And ofc no hard feelings xD and sorry if I missed something else.