On the basis that in a court, they would be considered reliable witnesses due to the position they held which gave them either access to pertinent information or specialist training and experience that made their personal eye witness accounts more trustworthy and less prone to misidentification than a lay person.
No, they wouldn't, because they didn't have such access. An astronaut, for example, no matter how well-trained or intelligent, is either a civilian (as were most of the Apollo crews), or a low-to-middle-ranking military officer. No way would a court consider them to be 'expert witnesses' in the fields of astrophysics, planetary science, xenobiology. And most if not all of the quotations cited are simply opinions, to which we're all entitled; but their opinions carry no especial weight.
Agreed that having intelligent high ranking military personnel/astronauts/politicians etc is no substitute for a saucer landing on the White House lawn with CNN HD live coverage but they have been privy to information within the US (and other nations) governmental systems that give them more credibility in their claims, as opposed to me or you...
Yes, but with a few exceptions they are not making claims. They are expressing opinions and without any cited evidence to back them up they cannot be assumed to carry any special credibility. It is notable that the three heads of state quoted all merely point out that they are setting up enquiries to find out more; nowhere is any hint given that they might know more than they are saying. And we don't hear anything of the results of those enquiries. Also, note that most of the quotations refer to a period of the 1940s and '50s where such ideas were new and credible to a wide range of the scientific community. This was the golden age of SF; Orson Welles; a vast increase of high-altitude aviation into uncharted regions where strange phenomena might be more common. The whole field of 'UFOs' struck a receptive nerve at the time.
Our galaxy is 100,000 light years across, a civilisation able to travel a tenth the speed of light would be able to travel the length of our galaxy in 10,000 years. Consider how much of our galaxy would have been colonised during a few million years.
True. But the practicalities are still immense. At the moment we are probably capable of building and launching a colony ship to the stars. But only one and that would bankrupt us even if we could find the collective will to do it. That's the biggest obstacle to the kind of expansion you envisage: simply deciding it's worthwhile and marshalling the resources. You could postulate new technologies but to be honest it's difficult to see how they could help unless we rewrote the laws of physics. To take an extreme example, for a galactic civilisation to be observable by us it would have to be no more than a few hundred light years away. How many solar systems must that civilisation have colonised first before reaching our neighbourhood, assuming it started across the galaxy?
I'm not ruling it out, by any means, but the probabilities don't seem good.