Question 1), could there possibly be any issue with leaving raw food on a flame before shabbos? Normally no issue with a melacha with setting it up ahead of shabbos, what about here?

Question 2) If your using a hotplate or crockpot is there anything you need to do to it ahead of shabbos to allow you to put food into it before shabbos?

Mishna teaches us that a raw food that has began cooking may not be left on an uncovered flame over shabbos. This is despite the fact that the act of positioning the food on the flame was done before shabbos.
This restriction is called "shehiyoh".

The reason our Rabbi's forbade this was that one might be inclined to adjust the flame in order to make it cook faster.

The general custom is to cover the range top with a sheet of metal called a "blech" before shabbos and even when the food is fully cooked.

The blech does 2 things:
1) it diminishes the heat of the fire
2) it serves as a visual reminder that adjusting the flame is prohibited.

On modern stoves one should cover the flame and preferably the control knobs too. A sheet of metal placed over the grates and burners is adequate.

Note that when a blech is in place you could even leave raw food on it, such a raw chullent which is placed there just before shabbos. When this is done, the pot may not be moved or tampered with until after the food is definitely cooked.

If you forgot to put a blech on, as long as the food is 1/3 cooked you can use the food.

It's best not to put uncooked food into an oven before shabbos to finish cooking. The only way to deal with this would be to cover the source of the heat. In most ovens it comes out of the bottom of the oven so only the bottom of the oven must be covered with a piece of heavy duty foil.
To add to this issue, if the thermostat is very sensitive a sudden change in temperature can cause the oven to ignite instantaneously. 

A crockpot or hotplate without a temperature settings doesn't need a blech, ie it doesn't require a piece of aluminium foil as there is no possibility that you may adjust the source of heat on shabbos.
But please note that for other reasons (insulating) it's best to line the inside of the cylinder of a crockpot with aluminium foil and allow the foil to extend noticeably higher than the cylinder walls.

However if the crockpot or hotplate have a temperature control it can only be used if the food was mostly cooked before shabbos, and the controls should be taped or covered before shabbos. For a hotplate, put heavy aluminium foil or a few layers of regular foil as a blech.

There is an exception to the requirement of covering the heating element when maintaining food on a flame from before shabbos; this is when raw meat is placed in a slow cooker where it cooks slowly and will therefore only be available for the day meal and not the Friday night meal.

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