Mine arrived today. How easy it'd be to read is dependent on how the letter is folded and put into the return envelope (which doesn't have your details).
The survey letter is folded originally such that if you return it the same way the Y/N part will always face outside. You could reverse one fold when returning which would put the Y/N part in the middle of the 2 other folds (standard A4 letter).
As for being able to read/distinguish what's been voted, it would vary depending on how the return letter is folded and which orientation it's put back in the envelope. Doing a dodgy check and chuck would be a pretty conspicuous operation for a manual worker and probably take a while.
For starters, the return envelope is just a generic one that looks like the millions of others that'd be in the system at any time.
Next up, like I said before - I doubt these things would be in the manual sort process at all. The system is fairly automated and most of the manual sorting happens in the number/street ordering by the actual postie that delivers the things on the day. Bulk return stuff like this needs no sorting at all and could be picked and binned at the earliest stage and IMO the automated process would get 99.9% of the things.
So, put the teacup back in the cupboard, the storm has found somewhere else to go.