Another long time lurker coming out of the woodwork, grateful for all your humour and insights over the years.
I can explain postal votes - worked in elections for many years. They are opened and verified in several sessions as they come in leading up to polling day, in the presence of candidates or agents if they want to observe.
Ballot papers are very strictly kept face down, while processes take place to ensure they are genuine, correctly signed, etc. We also try to match those that have got their paperwork mixed up in households so they don't lose their vote. This would all take far too long at the Count.
At the Count, they are mixed in with the other votes for the ward before being counted. So all we know before the election is the number sent out against those returned. A huge number of voters hand deliver their postal vote to the polling station on polling day - so you learn very little about anything from postal votes in advance. Although it's still a nice buzzy atmosphere!