He probably couldn't survive
In terms of the toxic atmosphere, we know that it wouldn't affect Data's 'respiratory systems' necessarily:
BASHIR: You're breathing.
DATA: Yes. I do have a functional respiration system. However, its purpose is to maintain thermal control of my internal systems. I am, in fact, capable of functioning for extended periods in a vacuum.
(TNG 'Birthright Pt 1')
So, he can just 'turn off' his breathing it seems. This leads to another matter though - Data SA's the purpose of his 'breathing' is to maintain his internal temperature - if he was subjected to extreme temperatures, he may have complications whilst operating.
Memory Alpha describes a Class Y planet as having the following characteristics:
a toxic atmosphere, surface temperatures exceeding five hundred Kelvin and thermionic radiation discharges.
This is a fairly high temperature, so Data may not be able to function at all. He has implied that his 'breathing' is an important function for temperature regulation - if internal temperature wasn't important, why would ---I--- Dr Soong have made temperature regulation a mechanism?
So, it's quite possible that he simply wouldn't be able to function at all in such temperatures. Modern-day computers require fans to keep them cool and those without them sometimes stop working when they get too hot. The same basic principle probably applies to Data.
As also mentioned in the question itself, the fact that even the EV suits were deteriorating suggests Data may also begin disintegrating. This may just be his skin, but that could lead to contaminants entering his systems.