I think it is normal. It is the absolute hashpower of the pool is growing, so your share is going down because its stable.
Yep. When the pool hashrate goes up other people are finishing the shifts before you can get the same amount of work in. That's why your work per shift goes down - the shifts are shorter time-wise.
I think he is concerned about the decrease in his hash rate from ~600 GH/s to ~560 GH/s which isn't affected by the pool hash rate. That change does seem a little large for normal fluctuation but not impossible. I wouldn't really worry about the "trend" as it is likely just chance. I don't have any suggestions but would definitely wait a few more shifts to see if the trend continues as it might just be normal fluctuations.
Aaron