Like I said (and I am Portuguese) you must either do the cob house without anyone noticing, or neighbors must remain quiet. If they snitch, then you will have to pay a fine and for the permits. Anyways, paying for the permits, which can be strict, implies money, which can be up to a 5k, because it requires an architect plan plus other expenses and taxes. And being a natural construction something that raises odd looks, I wouldn't go down that road. You need to access how much remote your place is.
Or you can speak to someone that knows the local council and establish a good relationship with them, and convince them of what you are doing, but that can be difficult and it would be nearly impossible if you are a foreigner. Anyways Portuguese are not as strict as rules, but everyone and the gov is in dire need for money, so they will use any opportunity to fine or make you pay something. And like I said, it depends a lot on the region and town hall.
You need to be realistic and know of these challenges and also the way how Portuguese people work. Selling mushrooms to restaurants... Hmm, I doubt it. You can try to sell them to organic shops, in large cities. But do not expect countryside folks to buy them. Guarda is quite a conservative place, people are very friendly, hospitable, giving and nice, but will give a odd look if you try to sell them a weird crop, unheard of (for them). Speak to them of permaculture and they will answer come and try my own wine or cheese
Cash crops: melons yes (they are a traditional crop), herbs (to organic shops or young city people, otherwise not so much), chestnuts (definitively but you need to do some street selling, it works best for chestnuts in autumn), wine and liquors (big yes!), olives are also traditional crop, beans not so much (but sold as bulk, can be, more countryside), cheese can be a good bet, tomatoes, peppers, garlic, turnips tops... Talk to your grandfather, he can probably come with good ideas. But as I said, difficult to make enough livable income, as you will compete with a lot, and cheap supermarkets.
Great that you have already irrigation, that's a big plus.
Guarda has also slighty colder winters than most Portugal. Snow can occur ocasionally and frosts can be down to zone 8. You can't, for example, grow bananas there, but still a lot of subtropical crops. Summers will still be nice and hot. Not so much problem with forest fires there as elsewhere.
On building the cob house, if you can have a large property, try to build the house in a remote place, and keep it low profile. If there is a ruin there, then it helps a bit. Laws can be flexible and bent but can also be a headache, just be aware of that.