Quinoa is a seed, not a grain, so the protein quality is actually quite different that that of, say, oatmeal, in terms of amino acid profile. I blend quick oats and quinoa flakes as a base for muesli, because it provides a better spectrum of aminos than oats alone.
The difficulty of vegan/vegetarian backpacking makes itself quite clear on extended trips, as Ben notes. Your body *will* absorb enough protein, but it will absorb it from your muscles, tendons, ligaments. The same with B12–the body stores some in the liver, but once it depletes that source, you'll become vitamin deficient, and a b12 deficiency can lead to depression, difficulty sleeping, low energy, etc.
As for a handful of nuts providing adequate protein for the day: 1 oz of almonds, eg, contains about 8 g of protein. A moderately active 120 lb person needs roughly 50 g of protein a day, but under physically demanding conditions should probably be aiming for 80-85 grams/day. I love almonds, but that's a lot of almonds (a bag of whole raw almonds from the baking section of the supermarkets is usually 10 or 12 oz and costs 4 or 5 bucks).
Protein intake will vary widely form person to person, according to metabolism, body type, etc, but those are the numbers I use for myself.