Mike

I am an engineer, and worry about such things to an extent bordering on OCD.
Your easy questions first -
If you are off by .1miles after 6 miles, then the error is 0.1/6 = 1.7%, your speed will be off by 1.7% too. Since your car spedometer probably doesn't read finer resolution than 1mph , this would mean that you would have to be going 60mph to have a 1mph error in the speed read.

Things that I have done to convince myself that my computer is set up right:
I will map out one of my routes using DeLorme Street Atlas - there are plenty of others too - map out a good long ride of 25 miles or more, then ride it and compare the number that you get on your computer to what the mapping software says that you should have done.

I have also ridden with a GPS unit attached to the handlebars and compared what the GPS says to what the computer says.

You can adjust the computer by changing the number that you load for tire size - i.e. if you loaded a number of 2125 for your 700c35 tires, and you found that the computer was reading 10%too fast, then you would decrease the number by 10%.

Or you can take the advice of my wife, and just not worry about it.