@MnM - looks you are Spaniard (guessing by the profile photo, of course I know it is not your photo.) Advice on some web pages seems calming, “just cooperate and it will be fine”. And this is quite scary (in Spanish): https://www.elconfidencial.com/economia/2017-07-10/asi-funciona-la-granja-de-ordenadores-que-esta-revisando-tu-declaracion-de-la-renta_1411090/ , but the tax agency has motivation to exaggerate its abilities. Moreover, I checked and I really paid taxes only in one year of the 3 audited years, another year I was just furious for all things I already wrote in “rant” section above, and another year I tried to compute my profit and it was negative (so it would be loss). I suspect I made some error in the calculation or forgot something, and even if not, they would not believe me.
Still not decided on a lawyer, I already have bad experience with Spanish lawyers, and hope that the taxman only wants to see some details and will be content then.