That's fine. So a few questions remain:
1) How many of the items you've noted were expressly *predicted* by the BB model--as opposed to being folded into the BB model *after* they were observed?
The story of how the model came to us is very interesting, IMO. It's not often one theory gets introduced (ie GR) and new ones come about from it prior to observations. Isaac Asimov noted that "the most exciting phrase to hear in science, the one that heralds the most discoveries, is not Eureka
! (I found it!) but 'That's funny...’
In the days of Einstein, the Static Model was all but cast in stone for all of science.; the universe was infinite, or beyond our ability to tell otherwise. I'm not aware of a competing model at that time.
So, when he began applying his GR theory to the cosmos he had no way to address the recently observed redshifts by the astronomer (a farmer boy) with one of the coolest names ever -- Vesto Slipher. The long, hard nights he had to endure to get enough photons to get a useful spectrum had to be very difficult and I read he had to lean into the telescope to help hold alignment. And all this from Flagstaff which was built primarily to look for Martians. [This may not even qualify as hyperbole.]
2) If observations were incorporated into the BB model *after* they were made, were there alternate models that would also have incorporated them with fewer "tweaks" (or no tweaks at all)?
Yes, and in once case no tweak was necessary, but....
So, it helps to recognize how impactful Einstein's General Relativity (GR) was in the world. When Eddington conducted one of the first tests of the theory regarding the calculated amount of bending of starlight passing near the Sun (during a solar eclipse) -- Newton's theory also predicted bending but not as much - and determined that Einstein nailed it, then Einstein quickly became famous throughout the world, along with this theory, of course. This prompted many brilliant scientists, including astrophysicists, to apply GR to the cosmos. Besides Einstein in GR clout was deSitter. They both had different models and both from GR, which isn't surprising.
Einstein's model was great but it "predicted" no redshifts since he was stuck on the Static model.
DeSitter's model held the Static model as well but did include redshift. But, he wasn't able to include any mass in the universe to make his equations work, apparently.
A little known Belgium priest (out of MIT), Georges Lemaitre, had left Europe due to WWII and was able to meet folks like Slipher and others who gave him great appreciation for the recent redshift results of Slipher. Once again, it was his knowledge of GR that was applied to the cosmos but he dropped the Static model since expansion was the best logical explanation for it. He published in French in an obscure Belgium physics publication and it was completely ignored. But Lemaitre had studied under Eddington and that relationship was used to wake-up Eddington, and the world, to his theory - The Primeval Atom.
Eddington was quick to recognize its importance and he soon got Einstein and others to recognize it, including deSitter who really liked it. It became somewhat famous and even the Pope tried to use it to help address Genesis, though Lemaitre sent a letter to him pointing out the problems with that idea.
3) What observation could be made that would invalidate the BB? Ancient objects older than 15 billion years? Local quasars? "Too evolved" distant objects? Evidence that redshift does not always indicate distance?
A theory this big can have many sorts of things falsify it. Only one falsification is needed to kill or injure a theory. There were two "injuries" two it that required tweaking. The Inflation Theory was incorporated to address the almost perfect isotropy found in the CMBR. This is still a bit controversial. It holds that the universe expanded well past the speed of light -- this part isn't a problem since space is expanding -- to a size about that of a grapefruit. This kept quantum fluctuations from producing greater anisotropy, for instance.
The list I gave all help affirm BBT. It's possible that something will be observed that kills BBT. Tired Light was supposed to, but it was respected for a while, but more studies diminished its likelihood.
Or would every new observation outside the already largely arbitrary boundaries simply be folded into the expanding universe model under the assumption that "some variable is missing"? In other words, what could *possibly* be observed that would cause cosmologists to seriously question the BB's fundamental premise?
I don't know but you can bet they are looking. Imagine if your that scientists that debunks one of the greatest theories of all time. Science is all about falsifying ideas to bring us new and better ideas. The Aristotle model got debunked by Galileo and the heliocentric model (first from the Greeks) has removed most if not all the capriciousness of those planets and points of lights going around us every day.
My point is that I don't think there's anything that could possibly be discovered or observed that would not be massaged into the model. That's very thin ice vis-a-vis the scientific method.
The more they look and find kinks in the armor the better the armor will become or new armor will be found. I respect all who do so, though I lack the skills to assist them since GR itself, as the foundation for any serious model, is very difficult and not something I'm interested in attempting to learn.
Personally, I think a static universe model is consistent with the observations, and it doesn't require the miraculous "breath of god" beginning to the universe.
You may not be alone, but it will be difficult to tackle many of those items listed. Redshifts were just the first challenge, but with the discovery of the CMBR, you will likely find many cosmologists who could help you revert to those good old days before Slipher and Einstein.
Yes, the BB is careful to avoid talking about that everything-out-of-nothing moment because of the mystical implications,
Yes, because physics itself falls apart when it tries to cross the first Planck unit of time. It is mystical, though many prefer to call it metaphsyics, though many also love to call them scientific theories since it is their theory they want everyone to respect and, often, buy there book. I am one who doesn't mind taking heat for calling them out, even if I am just an amateur since I see the need to keep science itself safe and secure by keeping within its self-assigned lines in the sand.
...but you can't get around the fact that BB proponents really do require a miracle that violates fundamental laws of thermodynamics before they even get around to discussing the scant details of the BB model.
This is indeed the realm of philosophy (metaphysics at best) and religion, which are subjective-based reasoning. I think many scientists stated that the validity of the BBT put a real dent in atheism.
As of for the laws of thermodynamics, physicists have stated that they don't apply so well to an entire cosmos, yet they aren't ignored either. The theory we have gone through many Big Bangs, after each collapse, has the 2nd law arguing against it and this seems to be a reasonable argument. It's over my head, admittedly.