1. Not sure, would be speculating otherwise. (Not saying that the other ones below aren't speculation either, but they're more logical explanations - which may or may not be true... )
2. Muscle wasting is pretty self-explanatory; muscles have a lot of proteins in them; so the less protein you absorb into your body, the less that can be synthesised for muscles and muscle contraction is therefore limited.
3. Oedema / bloated abdomen - I guess this may be to do with tissue fluid. If you absorb less protein, less protein, which is in blood, can be synthesised. This means that there is a smaller osmotic gradient in the bloodstream than in the tissue fluid, so water would move out of the the blood and into the tissue fluid by osmosis.
4. Poor absorption - guess it's because many nutrients require proteins in order to be absorbed into the cells / bloodstream, e.g. ion channels, so the less protein you take in, the less that can be synthesised for these proteins.