Their fossils exist.
These were birds indistinguishable from modern eagles except for one major and decided difference.. . . teratorns had wingspans up to 45 feet and weighed an estimated 450 pounds!"
Sure, I "get" that your anti-science enthusiasm frequently overwhelms any respect for facts, but let's begin here:
The largest Teratorn ever found is called Argentavis, with wing-span max of 26 feet and weight up to 176 pounds -- less than half what you report.
Argentavis lived six to eight million years ago, not 11,000 years ago:
Teratorns from 11,000 years ago maxed out at 14 ft wingspans, weights around 35 pounds.
Today's largest flying birds, Kori Bustards reach up to 9 ft wingspan & 44 pounds.
Swordmaker: "The largest modern birds capable of flight are the Albatross with an up to 11 foot wingspan and weigh only 30 lbs. or some reports offer the Andean Condor with a reported 10.5 feet and 25 pounds."
Kori Bustard in flight:
Swordmaker: "Biologists and Aeronautics engineers tell us that a scaled-up eagle with muscular engines simply could not fly under its own power without a complete redesign of its "airframe and power plant" . . ."
They also tell us that bumblebees can't fly.
This proves nothing about those critters, only that some aeronautical engineers are freeekin' idiots.
Swordmaker: "...the Cretaceous had flying dragonflies, completely indistinguishable from modern dragonflies, except the Cretaceous model had FOUR FOOT WINGSPANS.
Such an insect could not fly under modern gravity. . . cube/square law, again. . . and its form factor would be far different at four feet than at four inches."
First of all, Meganeura's wing span was two feet, not four feet.
Second, it lived in the Carboniferous period (330 mya), not the Cretacious (100 mya).
Third, oxygen levels are said to have been higher during the Carboniferous, allowing insects to grow larger, but also, there were no predators to eat dragonflies in those days.
Finally, you need to utterly reject whoever it is that's telling you all those critters "can't fly".
They obviously did.
Swordmaker: "Either these animals magically defied all laws of aerodynamics or somehow magically broke the cube/square law, or something was drastically different about the environment in which flew undoubtedly flew a mere 12,000 years ago and back in the Cretaceous."
No, far from it.
According to "Occams razor" theory, the simplest and most obvious explanation is best: whoever is making up these "laws of aerodynamics" are freekin' idiots who have not the foggiest clue what they're talking about.
And whoever endlessly repeats such nonsense is obviously driven by an agenda which has nothing to do with science.
Swordmaker: "...anomalies from those eras, such as your 40,000 pound pachyderms that NO LONGER, and can no longer exist today."
But not because of any stupid "laws" you fantasize, rather because they were driven to extinction by changing climate, predators and/or diseases.
Swordmaker: "Megafauna exists nowhere on land today yet millions of years of evolution seems to show that mega size is a survival factor.
Why not today? What has changed?
The best answer seems to be gravity increased."
But that's not the "best" answer, it's the stupidest answer imaginable.
Any other reasonable explanation (i.e., climate, predators, diseases) better fits the facts as known.
Swordmaker: "...blood the mere SEVEN FEET from a Giraffe's 24lb heart to its brain is so high—300 over 200—that it would most likely cause any other animals' arteries to blow out before long!"
Unless, having evolved over many, many generations, heart, arteries & other pumps grew robust enough to accomplish the purposes.
Swordmaker: "Yet what was the size and weight of the blood pump possessed by Argentinosaurus—which weighed an astounding 400,000 pounds, was 125 feet in length, and had a 65 foot long neck—to have blood reach what ever sized brain it used?
The mind boggles thinking about it."
First of all, Argentinosaurus weighed 100 tons, not 200 tons.
Second, it was about 100 feet long (1/3 neck) and 24 feet high.
So typically, its head did not rise much above the level of it's body & heart.
Third, there's no "mind boggling" required, since all such critters had pea-brains requiring far less oxygen-rich blood than we assume today.
Speaking of oxygen -- note the geological eras' oxygen levels higher than present:
Swordmaker: "NOW, do you begin to grasp the problem?
This is an issue that simply cannot be swept under the proverbial rug.."
Yes, I see clearly that you sell blithering idiocy, that this cannot be swept under the proverbial rug, and you and your buddies should all seek out professional help with it.
Otherwise, there's no hope for you...