To address your points (not in any particular order):
1. Not so much what happens to the chloride ion, its the fact of the body being able to do it quick enough to maintain state Vs. what we're infusing. Infusing considerable amounts of NS on its own has been known to do this sort of thing, but we have to remember that there are ALOT of drugs out there that are chloride salts. (Things like Ringer's lactate, if my mind serve me correctly are basic and will mitigate the acidosis.).
Your mind does not serve your correctly...
2. Its basic theory CO2 causes acidosis when it builds up- CO2 itself is an ACID. Using Bicarb during an arrest was old-school protocol under the notion of correcting the acidosis caused by CO2 however, its fallen out of style in practice. (most probably because acidosis hasn't been systematically shown to correlate to isoelectric function of cardiac cells directly- though I'd tend to think that letting cells sit in their own waste product doesn't help things much.) However, if I remember correctly it has remained in the ACLS guidelines over the years, though anymore tends to be used after the code has been running a considerable length of time.
Bicarb does not correct acidosis, ventilations do..
3. Even if our patient has good kidney function we certainly don't want to do anything that tends to promote renal failure.
Mag does not cause Renal Failure, but is the result of kidneys that have failed..
4. Decreasing metabolic demand is probably a discussion thread in and of itself. One we're not having today.
5 . If we were to use bicarb to neutralize mag, we would end up producing significant heat without having a catalyst to control the speed of the reaction. And most likely will still end with unreacted bicarb - assuming that we don't consider that the two are likely going to react to form a third salt.
Why would you use Bicarb to "neutralize" Mag
6. With reference to your point as to a mild acid altering serum pH it's not as straight forward as it appears. It depends upon (A) what the serum pH is to begin with (as the more acidic a solution is the less buffer it has) (B) How many hydrogen atoms are involved in the equation. among other things. Your probably not going to be able to work a chemical equation out to enough precision without doing the kind of math that is a leading confirmed cause of migraine headaches.
WHAT??