No, it’s not calibrated, it’s directly measured. And if it changed, then either God caused a pointless and deceptive miracle, atoms ceased to exist, or the Earth turned into a ball of radioactive plasma.

relies on spurious assumptions
It relies on the assumptions that reality is real and is objectively knowable, that our senses give an accurate view of reality, and that measurable physical constants with observed effects on properties of the universe do not magically change with no evidence.

they confirm what they believe, it’s called confirmation bias.
I don’t know of anyone who thought that pterotracheoids were closely related to littorinoids, and yet there are at least three research groups within the last five years who have found that based on statistical analysis of DNA sequences. I also don’t know of anyone who thought that Plesiotrochus had a Quaternary geographic range including the Carolinas, and yet I’ve seen two specimens of it from there.

But I wonder how many of those scientists, if pressed without possible negative consequences of perhaps losing a large research funding grant
Given that I work in a field (systematic invertebrate paleontology) where a big grant might be a few tens of thousands of dollars, and occasionally someone in the field will manage to get a grant, that’s not an issue. It’s also a slander against scientists in relevant fields.

losing the prestige of having one’s research paper published in say ‘Science’ or ‘Nature’ or any of 100 other journals, would still maintain that they aren’t making any assumptions about dates of say fossils for instance,
I just stated the assumptions made, and each one is also required for knowing that anything in the past existed at all.

in real unmineralised dinosaur bone to be 66+ million years old?
That’s still a lie; the bones and tissues have been mineralized.

easily repeatable straightforward laboratory experiments clearly demonstrate that the established laws of both physics and chemistry are severely violated when the soft tissue found in real unmineralised dinosaur bone are loudly proclaimed to be 66+ million years old?
What “established laws”? The same ones that say that radiometric decay can’t change speed?