All canned foods have been pre-cooked, so it is not necessary to heat canned food from a safety point of view. Heating up the food will enhance the flavor of the canned food and make it feel like a heartier meal, but this is not necessary. Canned foods can be eaten cold, right out of the can.
Leeches are prevalent in many parts of the world and can be eaten for survival. Eating live leeches can be done, but they pose a danger if they are not chewed thoroughly. The best way to eat leeches is to grind them into a paste and fry them in a little oil or butter.
When things rot, the bacteria that cause decay proliferate on the item and begin to break it down. Fermentation is the introduction of good bacteria that inhibit the growth of the bacteria that cause decay. This has the effect of preserving the food item for longer, and even enhancing its nutritional value due to the presence of the good bacteria!
You can eat a dead crawfish or lobster if it has not been dead for long and if you harvested it yourself. Harvesting it yourself, you would know that it was alive when you caught it, and it can be eaten in a short time after. If refrigerated, a dead crawfish or lobster can last a day or so. Dead clams, on the other hand, should never be eaten if they were harvested dead or died in transit to your cooking area!
You can live off oatmeal alone provided that the duration you only eat oatmeal is not too extended. Oatmeal cannot sustain you long term by itself; you would need to supplement it after a while with other foods to get a sufficient energy-rich nutritious diet that will sustain you with enough energy for survival.
It is definitely possible to survive on rice and beans alone, since the meal is considered to be a complete protein, providing all 9 amino acids that are essential to life. The carbohydrate value of rice provides a good source of energy. Rice and beans partnered together will make a survival food that can sustain you indefinitely if you can stand the boredom!
All dehydrated foods will expand to some degree in your stomach. The degree to which they expand will depend on the dehydration process and the type of dehydrated food. It is, however, unlikely that the extent of the expansion will be the cause of any health concerns other than a little bloating.
While there is no doubt that coconut milk has nutritional value and will help to stave off dehydration, you will not be able to last significantly longer on coconut milk as opposed to water. Coconut water is a mild diuretic, so you cannot rely on it alone for hydration. You will need an additional hydration and energy source to meet your daily needs.