You can ask question “What is percent digestibility of plant protein?” here http:///Q/What_is_the_percent_digestibility_of_plant_protein
The answer is 70-90%
In relation of most animal proteins it is 90-99%. So the plant protein is less digestible, but this difference is not catastrophic. Some protein from plant and animal sources have same digestibility – 90%. So, why you making big problem from nothing? You write about the cellulose as it is on the way of digestion, but it is not meant to be digested, only bulk the feces, provide environment for friendly bacteria, and brush the intestine. Do not panic, vegetarians and vegans, you are save enough if you eat wise. And what to eat is personal choice, based on personal experience and knowledge that is always limited. People, study yourself and live in peace.