Quite a Gish Gallop you got going there. Full of unsupported assertions and errors. I'm particularly irritated by your quote from someone who has no clue how radiometric dating works, just as you have no clue.
1- A constant rate of decay is assumed
Sorry, no. A constant rate of decay is concluded from vast mountains of evidence. Even the RATE group acknowledges (and tries to minimize the importance of) the show-stopping problems with any theory that tries to introduce non-constant decay rates. Decay rates are a consequence of some very fundamental properties of the Universe; if they changed all sorts of other things would change too ... and those other things haven't changed.
a) The constancy of cosmic ray bombardment might be questioned. The current high rate of entry might be a consequence of a disturbed post-flood environment that altered the carbon-14 to carbon-12 ratio. Pre-flood dates would thus have to be discarded.
Let's see exactly how this would work. Hint: it wouldn't. Show the math.
b) An increase in the magnetic field of the earth would have shielded the earth from cosmic rays. Some scientists argue that the magnetic field of the earth has declined over time.
No scientists argue that the magnetic field of the Earth has decreased. A few loons do. This is another PRATT.
Carbon dating is calibrated against other methods. Other radiometric dating methods are not affected by cosmic rays.
c) Atmospheric carbon forms just 0.0005% of the current carbon reservoir-99.66% of the earth's carbon exists in limestone, 0.31% in oil and gas, and 0.02% in coal. carbon-14 comes from nitrogen and is independent of the carbon-12 reservoir. If even a small percentage of the limestone deposits were still in the form of living marine organisms at the time of the flood, then the small amount of carbon-14 would have mixed with a much larger carbon-12 reservoir, thus resulting in a drastically reduced ratio. Specimens would then look much older than they actually are.
Let's see the math. Especially show how this "model" explains the dating of objects of known age, such as the bread found in Pompeii. Hint: it would show that the bread was baked from wheat grown centuries after Vesuvius erupted.
d) Even if the rate of decay is constant, without knowledge of the exact ratio of carbon-12 to carbon-14 in the initial sample, the dating technique is subject to question.
That's why the method is calibrated against other methods.
2- It's assumed that the clock was set to zero when the study material was formed. This requires that only the parent isotope be initially present or that the amount of daughter isotope present at the beginning is known so that it can be subtracted.
Not relevant to carbon dating. For geological dating methods, it's just false. For example, even the RATE group acknowledges that zircons form with no significant lead content. Argon-argon and isochron dating produce the amount of initial daughter product as a byproduct of the method.
3- It is assumed that we are dealing with a closed system-no loss of either parent or daughter elements has occurred since the study material formed.
False. The most commonly used dating methods indicate when the system has been open, and most of them often yield a valid date even when the system has not remained closed.