How could it be that this gold coin was created as legal tender when its gold content value is much higher than its face value?
https://en.wikipedia.org/wiki/Canadian_Gold_Maple_Leaf
The face value is 50 Canadian dollars. 1 ounce of gold is currently about USD 1,300.
Is it actually being used in Canada as legal tender?
How could these coins have remained in circulation and not being melted down by everybody?
Just because the coins are legal tender does not mean that they are "in circulation" and people are spending them at stores.
Yes, the Canadian Gold Maple Leaf is legal tender, as is the American Gold Eagle. All this means is that if you owe a debt of $50 to someone, you can give them one of these coins, and they are required to accept it as payment in full.
However, the coins have an intrinsic value that is much higher than the face value. They have this high intrinsic value because of the gold content. It would be foolish to give them to someone in exchange for $50 worth of goods.
To answer your question, they could be melted down and have the gold harvested (illegally). However, doing so would decrease the value somewhat. The gold content contributes to the value, but they also have value as collectors items. The coin form also helps specify the purity and weight of the coin, and if the coin is melted, the resulting gold lump would need to be tested by a buyer to verify the purity and weight. As a result, these coins don't get melted, and people needing gold for utility purposes don't buy coins and melt them, as it would be more expensive than just obtaining raw gold.
How could it be that this gold coin was created as legal tender when its gold content is much higher than its face value?
There are many coins that have market value far above their face value, typically collectibles of limited production. They are often issued to create revenue for the treasury. You can't buy one of these for 50 dollars.
Is it actually being used in Canada as legal tender?
Only foolishly, if at all.
How could these coins have remained in circulation and not being melted down by everybody?
They're worth more than their gold content alone, they seem to sell on ebay for about USD 1,400, so there'd be no motivation to melt them down, you'd just sell them at market value.
This kind of coin is issued because:
The second point is hard to understand given how low the fiat value is compared to the commodity value of gold. And over time, I would expect that gap to expand with inflation. Unless someone finds a way to turn acorns into gold or something, I can't see how the face value will ever matter.
