Let me take a stab at these....
1. Is it feasible to have a harvest in continuous process such that I dont have to buy from the store, or am I going to have barren periods in the year?
If you set up like a commercial mushroom farm, like the caves in Pennsylvania, you can have continuous production 365 days a year. If you keep the temperature and humidity constant (light doesn't matter) and bring in new media and inoculate it regularly, you can get consistent results.
2. What land area do I need to set aside for this project?
Land isn't the limiting factor, in commercial production, boxes of media are stacked with just enough vertical separation to have some air flow.
3. Are there any prerequisites for this land (I am hoping the answer is no,you can just tuck it under a shrub/tree layer..)?
You could tuck it into a cave. These boxes of
Agaricus campestris that you can buy mail order, they will produce 2 or 3 good flushes in a basement or a crawl space before they are spent.
Let me add that traditionally, mushrooms are a fall food. In the wilds of Northern Europe, fall was the time when all the plant life was getting ready to shut down for the winter and the fungi would sense the coming scarcity of food and send out fruiting bodies (mushrooms). People would collect during this season of surplus and then dry them for use in winter and spring, the barren periods. When you get further south, into Mediterranean climes, it's less about cold winter and more about the rainy season (Oct.-Mar.). There, you could look for a flush of mushrooms after a heavy rain.
Western WA is an excellent location for this endeavor (very rainy) and you might want to consider inoculating with some reliable species that can persist in your environment. I have lots of oaks on my property, and they are pretty reliable for producing
wood ear mushrooms on the dead branches in the winter and spring. You may want to do an inventory of your trees and find out which will support varieties of edible mushrooms, and if they are not present, get some and inoculate your trees. But one caution if you like maiitake mushrooms; they are not just saprophytic on oak trees but also pathogenic -- they will kill their host oak tree after a few seasons on it.