Some "living fossils" just don't change their appearance but change genetically

I didn't know the mechanism only that organic Mercury molecules could pass the blood brain barrier easier.

Metallic Mercury is the least dangerous one Mercury salts are deadly Organic Mercury is the worst of them As a generalisation for oral consumption mechanism

If it is air tight it shouldn't be a problem, the high vapor pressure makes that difficult if the temperature changes. You should look up proper storage procedure. I was told dangerous substances are only dangerous if they are where the shouldn't be. Poison that looks like candy is totally save if placed in a locked poison shelf. Dangerous if left in the candy shelf.

the term "mad as a hatter" comes to mind, hat makers used mercury salts in their work.

metallic mercury isn´t that toxic in the sense of poison or venom it is mostly harmfull because it is in the way. you could drink mercury without adverse effects if you don´t inhale. it was used against constipation.

if you inhale Mercury it stays in the lung and blocks area that normally used for gas exchange.

If it is warm blooded and it's gets to big it basically cooks itself because it can't get the heat away fast enough. Growing and heat regulation come hand in hand during evolution other wise it wouldn't survive. Inanutshell has two videos on YouTube about the effect of size on lifeforms. Like shrink an elephant to the size of a mouse ind it would freeze to death grow a mouse to the size of an elephant and it would practically boil alive. And I heard in a documentary that the megafauna back than might have been pseudo warm blooded.