All companies do not use the same fillers. The Puritan's Pride ones are very different IMO. The inert ingredients are mostly
to improve flow of the powder and are not absorbed if at all.
Puritan's has various vegetable cellulose, mannitol, magnesium stearate (not absorbed), natural cherry flavor.

Not much to react to.

No one can say for sure because everyone is different. But if you notice a significant difference between methyl B12 and cyano injections, then I'd question your ability to convert cyano well in the tissues.

There is always up and down with B12 in some people. The tissues take it up differently and the movement into the brain and spinal cord can take time. Also the liver is storing up extra from each dose for the future.

People who are severely anemic may experience some low potassium as the extra cells gobble up potassium from your other tissues. So eating a potassium rich diet may help you.
A can of V8 juice has over 800mg of potassium.