Hi, AnticoUomo, First I want to send my sympathies to you as well, the death of a child must be an unbearable tragedy. I hope that walking the Camino will bring you some comfort and strength.
In terms of your passport situation, do you not have a return plane ticket?
This is one of those situations in which the rules and the reality MAY differ. First of all, no police officer in Spain and no hospitalero in Spain will be looking to see the dates on your passport. That's an immigration question, and it's the immigration officials at the airports, maybe on trains sometimes, etc, who will be the ones with the authority to question you and who know the visa rules. The others are just using it for ID.
I don't think anyone is trying to help you commit a crime, but rather just describe the risks you will be taking. You know what the law says, and you will be violating it by staying in the EU for more than 90 days.
My son once stayed for a year in Spain without a visa. He even left Spain during the year period for a week trip home to the US. At the airport going out, the immigration official asked him about why he didn't have a visa and told him it could cause him problems, but that official didn't cause him any problems (you have to wonder whom the immigrations official was referring to if not himself!) and a week later my son returned to Spain and finished out the year. Again no problems, and this time no problems at the exit. This wasn't a smart thing to do, he could have easily gotten a visa since he was studying, but being a stupid young male he didn't. We didn't know about it at the time and assumed he had gotten all the visas through his university program but he hadn't. I certainly don't recommend it, don't condone it, but that was his experience. As falcon says, though, this lax attitude may change at any moment (his experience was about 4 years ago).
As others have said, your main risk is when you leave the country. I know stories of people who have had problems at the exit, they weren't as lucky as my son. If you transfer at London, you should be prepared for a real problem. If you are going straight back to the US without a European transfer, it will depend on the attitude of the immigration official who looks at your passport as you leave Spain. In hindsight, it's clear what you should have done, but you didn't do it and now the rest is up to you since you're already there. Like leaningforward says, for me it would be a huge cause of anxiety, but since you have no way to get a visa to authorize an extension, I guess you'll have to decide how risk averse you are and go from there. Being banned from the EU for five years would be a horrible thing for me, maybe not for you. There is no good decision here, I suppose that's the real kicker. Buen camino, Laurie