All of the above.
This is not different than a money transfer, where people can enter a custom message. You get data into your system from an untrusted source and you have to be smart enough to sanitize it.
It's amazing how many people mess up the date calculations in their unit tests and forget about leap years and daylight saving time.
Mandatory xkcd reference: http://xkcd.com/400/
I don't know why I bother replying, but anyway.
The gate addresses do change with galactic drift. That's why in the Stargate movie they could only connect to a single gate. Normally the DHD takes care of recalculating the address, but they didn't have one on earth.
The ninth chevron is for selecting a location that is even farther away than a single galaxy. The stargate didn't connect to the ship while it was in hyperspace: if you watch the intro sequence, you can see that the ship drops out of hyperspace before the wormhole connects. After the gate was closed, it entered hyperspace again.
As for the death of the senator: they just wanted to get rid of him
You forgot the link.