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As far as your question on mechanical digestion goes...you are right in saying churning is an aspect of stomach mechanical digestion. Here is why it is important. Imagine whatever food you ate, chewed, & swallowed sitting inside your stomach as a solid ball. When the gastric acid is secreted from the walls of the stomach, the HCl can only work on the surface area of the ball, leaving the interior of the 'food ball' unaffected. Churning and mixing of the food solves this problem by mixing the gastric acid with the food, and basically increasing the surface area available to gastric acid.
Now chemical digestion of the stomach. The main chemicals to pay attention to here are gastric acid (a mixture containing HCl from parietal cells) and pepsinogen (from chief cells), both being secretions from the stomach wall (but from different cell types). Gastric acid plays a two part role in the stomach. First, it kills bacteria in the food by exposing them to a highly acidic environment (even more reasons for mechanical digestion) and second, it lowers the pH of the stomach contents enough to activate pepsinogen (pepsinogen's active form is called pepsin (usually the suffix -ogen denotes a precursor molecule). Pepsin's job is to start digesting proteins in the stomach. (There is very little digestion of much else other than proteins in the stomach, although startches (like bread, etc.) starts being broken down into sugars as early as the mouth by saliva components.Pepsin breaks food proteins down into peptides, which are small chains of amino acids (your building blocks of all proteins).
The liver and gall bladder do not come into play until the small intestine, after the stomach. The gall bladder is a gland in the liver that stores bile, which is produced by liver cells (Hepatocytes) & moved to the gall bladder, where it waits to be moved to the sm. intestine. Once again, it has more than one role. Its first role is to raise the pH back up from its acidic state achieved back in the stomach (a big portion of bile is a watery base). The second is to help with digestion of fats. Basically, we can use the same analogy we used earlier to explain mechanical digestion. When fats are present in chyme, they exist as big blobs that stick together (think of oil in water). The salt portions of bile act to break up these big blobs into smaller and smaller blobs, until they are small enough to be absorbed, more or less, through the small intestine wall.
Phew.
Hope that helps
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