1. The exponential distribution describes the time until a block is found. What you should be looking at is the number of blocks per time period, which follows the Poisson distribution, for which the variance is equal to the mean.
2. Variance / mean is not a very meaningful quantity. You should be looking at Variance / Mean^2.
3.
Let
H = pool's percentage of the network's hashrate
h = the miner's percent of the network
N = the average number of blocks per day (about 144)
B = Bitcoins per block (about 25)
T = number of days in the period we are looking at
Then the miner's average income is hNTB, the variance is h^2NTB/H, which means that the relative variance is 1/(HNT).
4. The network hashrate can be calculated as about 7 million times the difficulty.
5. This is all covered in
AoBPMRS.
6. The real answer to the variance counterargument is not "there isn't that much variance in small pools", it's
Multi-PPS.