Um, I'm sorry but I'm having trouble understanding you; I'm not really sure what you're arguing/saying. Hopefully this is actually an appropriate response:
Late-game sweeper is once thing, but why would you reveal your Volcarona in the beginning, especially with SR and all of the much faster Pokemon running around.
I think we agree. Like I said, it can't do much/is really restricted early on in the match.
After all of the threats are wiped out, then 5-% SR damage is still bad, but it isn't an instant death sentence for it.
Well, of course. "After all the threats are wiped out" is an argument for any Pokémon. "After all the threats are wiped out", Delcatty can sweep. The thing is, there are a LOT of threats. Volcarona's SR weakness means there are a LOT of threats. This is why I said I think it's best as a late game sweeper, and even still, there may be a few Pokémon that are faster or strong enough to finish it off when it comes in that weak.
Morning Sun is a somewhat reliable recovery, and as you said, why would Volcarona stay in on Sand and Rain?
Um, Sand and Rain are permanent? If you're facing a sand or rain team, you're playing the match in the sand or in the rain, period, unless you have your own weather. What I'm saying is, the Pokémon on a SS team (many have Rock type moves for starters) and a Rain team (combined with rain's weakening of Fire moves) makes Volcarona pretty difficult to use in these conditions anyway, so it most likely won't even get a chance to use Morning Sun, even if it could heal more than 25% under Rain and Sand, you know? So what I'm saying is, you aren't missing out on healing there, since you're pretty much dead anyway, and Volcarona will most likely not waste time trying to heal itself only to get murdered by Excadrill's Rock Slide.
Thus, I think Morning Sun is better than ChestoResto since you get to be healed multiple times as opposed to just once, even if it is restoring full health and removing status. The benefits don't outweigh the costs there in my opinion.