Okay, so I decided to take another look at the team and actually just very quickly theorymon'd out something that can do pretty much everything your Roserade is doing but can also check a +1 Volcarona: Defensive Venusaur.
(M) @ Leftovers
EVs: 244 HP / 56 SDef / 208 Spd
Calm Nature (+SDef, -Atk)
- Sleep Powder
- Hidden Power [Fire]
- Leech Seed / Energy Ball
- Protect / Substitute
With this EV Spread, Venusaur hits a stat spread of 362 HP / 202 Def / 235 SAtk / 273 SDef / 247 Spd, compared to your Roserade's 313 HP / 177 Def / 331 SAtk / 275 SDef / 216 Spd (i.e. it gains 49 HP / 25 Def / 31 Spd).
It does lose out on a lot of SAtk but this set is more focused on crippling threatening Pokemon with Sleep Powder and dealing passive damage with Leech Seed than hitting hard. The Spd investment allows Venusaur to outspeed +1 Timid Volcarona in the Sun,
(factoring in the 30 Spd IV for HP Fire) allowing you to Sleep Powder it (assuming no Chesto which is only at 12% usage anyway), and then either Leech Seed it or go to a better check. There is probably a way better EV spread out there, but I just wanted to show-off the defensive stats Venusaur can reach compared to your current Roserade, and still have enough EVs leftover to check one of the biggest threats to your team.
Bear in mind I've never used anything like this outside of sun, and certainly not on a team with its own sand-inducer.
It probably won't be nearly as good as I've hyped it due to clashes with Hippowdon's Sun and losing out on Natural Cure, but I think its at least worth a shot considering the better overall bulk it gains and the ability to check Volcarona which you've highlighted as a big threat, whilst still being able to absorb Toxic Spikes, switch into Rotom-W and maim Scizor.
Still, I think Toxic Jellicent and Rock Slide Hippowdon are much more preferable changes to deal with Volcarona, but I just suggested this to give you another option if you don't like what anyone else has suggested.