I think the first thing to say is that it sounds like you've had an awful time, and you are quite rightly questionning the events and procedures which brought about the suffering you and your family have endured.
I think it would be innapropiate to respond to the individual cases you have highlighted, but perhaps it may be useful to bring some insight to the situation generally.
Incidentally, a simillar thread is also currently running.
http://www.pprune.org/forums/showthread.php?t=299823
I'll deal with 2 issues you've raised.
Firstly screening: your quack is as good as his toolkit. Unfortunately the tools he's got to hand are generally pretty limited, and unfortunately do not always give the result we desire. Screening tools have to be acceptable to the population they are used on. They have to be accurate, and the result has to make a difference to the eventual outcome. Otherwise, they are not useful as a screening tool. Unfortunately, as the science is at present, 2 of the 3 tools you mention, do not fit these criteria.
Secondly, access to investigations. Here's how it works. The patient presents with a set of symptoms, the GP has access to the specialists, tools, scanners etc.
If he's worth his salt, the Gp will be taking into account all the factors in your case, the story of the symptoms, pre-existing illness, family history, results of his examination etc. IHe will then be making some sort of risk assessment, how serious, or how potentially serious is this situation. The results of this assessment will then guide his management of your situation.
Sometimes, your problems fit into a neat little "guideline," and the next step is quite clear. For example, you were 60, and you told me that you had lost 3 stone, had heartburn, and your food stuck when you swallowed, I'd be wanting to get you investigated, (a camera in the tum), pretty much straight away. (In the UK, the "journey" between you presenting to your GP, and being seen by a hospital quack should take no less than 2 weeks).
If you told me that you were 20 years old, drank 8 pints of
Stella prior to your chicken Jalfreizi, and woke up with heartburn this morniong, I'd be a little reluctant to send you off for further investigating. (In that case, the chances of you having something nasty, are about one in 250,000, the chances of the camera in the tum causing harm are about 1in 1000, of serious harm, 1 in 10,000.)
Unfortunately, most patients seem to fit somewhere in the middle, which is probably why a machine will never completely take over from your quack.
I wish I could tell you it was more magical than that, it isn't I'm afraid.
(Although others may have a different perspective.)
Cheers, ginge.