Speed cameras as far as I know have a tolerance... but that is because they NEED it, because they MAY be inaccurate.
They need to allow you to be going 103, for eg. because they can actually be reading 103 when you're only going 101 or 100 - so if they "snapped" you for going 103 they would be incorrect.
Ie. - u may go past one speedcamera and get flashed going 105 because it thinks your actually going 105, but NOT get snapped by another in a different setup even though your going 105, because it only thinks your going 103 say... (which is in it's tolerance)
^Pretty sure on all this anyways......
1. The speedo is aimed to be showing slightly faster than your actual speed. The reason for this is that if they set ALL cars to try and show the exact speed, ALL cars are slightly different and will actually read a little higher or lower depending on wheel size / inflation and MUCH more....
BUT it's only aimed that way. If your car has done a few k's, u've wacked 19" 's on it etc. there's always a slight chance you'll read UNDER and think you're going 100 when ur going 103 (use a speed-check point if ur curious I guess?) For most it will read slighly higher though, if not near-perfect
2. U don't get a warning with speed cameras (u can only bargain with cops/radar guns... speedcamera's either they got u and ur fined or they didn't)
3. It'll depend on your insurer. they don't mind you asking - I always call before I make any changes to my car and get a new quote with the new part on BEFORE I make any changes.
You can always ask, doesn't mean they're going to go "oh, why, thinking of speeding are we? ur premiums are going up kiddo!" lol

(how lucky they don't do that!)
4. I've pinned like 4 points on my mum (in one hit, I had like 11- lol) and it was easy as BUT it was a company car and we both work for the same company... (Note - obviously this can only be done with SPEED CAMERAS, as a cop will take your drivers liscence etc.
U have to fill in a part of the speed-fine form that says "it wasn't me it was..."
U could try and say it was ur old man maybe, they do have a photo - so if they see an obvious male and u say it was ur mum they MAY call u up on it...