Have I really legally been served if there was just a stack of papers laying and blowing at the end of the driveway by the street?

It doesn't sound like good service, but if you ignore it some judge may disagree with you, and you might find yourself with a judgment against Dad. Sooner or later, the plaintiff's lawyer will figure things out, and you will get served as trustee. You might be better advised to look at your insurance policy, to see if this sort of thing is covered. If so, let them handle it. If not, get a good civil defense lawyer with experience in this field.

The short answer is "no, there is no proper service here." HOWEVER, there is a practice known as "sewer service," where less-than-honest process servers will do something similar to what happened here, and then sign a proof of service saying service occurred. This will lead to a judgment being entered without your even knowing about it. You will then have to spend money to have the judgment set aside (and believe it or not, the effort to set the judgment aside is not always successful). I have seen variations on this problem twice with clients in the past two years. Best practice would be to reach out to a litigator (a lawyer who does courtroom work) and start dealing with this issue. It will save you money in the long run. This is particularly true because you have notice of the problem. The Court will take into account how quickly you act in addressing this problem once you have notice. If judgment has already been entered (you may be able to find out from the Court's website), do not fret. Go immediately to a lawyer and file a motion to have this set aside. This isn't not something a lay person is generally able to do by himself it's not simply a matter of explaining orally to the judge what happened. Indeed, because you were not personally the defendant and there is a trust involved, you may be forced to hire a lawyer. Again, do not delay here even if there is no judgment yet time is not your friend in this situation.

Yes, after attempting personal service a number of times, leaving documents with a person over 18 at the last known address can constitute valid substituted system. It sounds as if your inherited property is subject to a shakedown ADA lawsuit. Settle it ASAP and move on.