Yes, Einstein is a good example of someone whose views were constantly evolving, and he frequently repudiated some of his earlier beliefs. He once said about himself (only half joking) that every year he retracts what he wrote the year before.
Sufficient for what? Generally speaking, most students don't need or want to understand the subtleties of the foundations of scientific theories, e.g., the circularities lurking in most naive definitions, and the difficult epistemological issues. So the fact that textbooks do an abysmal job of explaining those things may not be a huge drawback, generally speaking. But what we're talking about here is precisely the foundational issues of a scientific theory, such as the possible meaning and significance of the concept of absolute rest in the context of classical mechanics. I don't think very many textbooks on classical mechanics do a good job of explaining that (nor, to be fair, were they intended to).
In contrast, original works by great thinkers tend to at least pay some attention to the foundational issues, because that's what those thinkers have been wrestling with in the formulation of their theories. But of course, being at the earliest stages, they often have confused and emerging ideas, and often change their minds later, so even the original works of great thinkers can't be taken as definitive statements of that individual's views (as you noted above) - let alone as statements of the truth. Many of the great scientists have been likened to "sleepwalkers" who were guided more by spiritual beliefs, instinct, and intuition than by clear-headed ratiocination.
Needless to say, there IS a vast literature on the foundational issues, but it is in books devoted to the foundations of science, and crackpots hardly ever seem to be acquainted with that literature.
Yes, although in most such cases I think the obscurity is more in the mind of the crackpot than in the original text. For example, Einstein says "for ease of reference, let's call this the stationary frame", and the crackpot says "Aha! Busted!" That's just the dementia of crackpots at work, not an obscurity in the text.
Of course, there ARE genuine obscurities and ambiguities in any text, but crackpots rarely notice those. This thread is a case in point. There are certainly obscurities in Newton's Principia, but on the question of absolute rest Newton was extraordinarily
careful to consistently equate "rest" with "uniform motion in a straight line", and he explicitly states the complete relativity of his mechanics. And the fleeting mention of absolute position and velocity in the Scholia is carefully labeled by Newton as purely metaphysical ("the places of absolute space do by no means fall under our senses") and religious ("God constitutes space and time"). In addition, he rather carefully discussed the difficult foundational issues in the Scholia, making the crucial case against relationism with his pail and rotating globes examples. All in all, an incredible intellectual feat - which makes it all the more regretable to have it mis-represented by crackpots.
I know metaphysics and religion fall withing the realm of philosophy, but my point is that science does too. Saying that Newton's concept of absolute rest was 'philosophical' doesn't really make the distinction you're trying to make (certainly not to antiquarian crackpots), because objective scientific thinking is philosophical too. Indeed, science in Newton's day was called natural philosophy
, and even today the modern dictionary definition of 'philosophy' doesn't exclude "objective" thinking "subject to the rules of science". It's true that some people (especially scientists) use the word 'philosophical' in a limited and (dare I say it?) even slightly perjorative sense to mean "metaphyical" or "subjective" or non-scientific, but that's a specialized meaning that only some people have in their minds. (I recall that Feynman used to delight in referring to the Phil-AH-zofers.) And of course the very distinction between objective and subjective is subject to objective debate within philosophy. So, if your concern is to avoid words that "people tend to apply differently depending on which view they personally hold", I think you should avoid vague and all-inclusive words like 'philosophical' that mean different things to different people, and instead use more precise words like 'metaphysical' and 'religious' to signify (respectively) ideas that have no observable physical significance (does not fall under our senses) or are based on religious beliefs (e.g., the sensorium of God).