I can find nothing that would indicate that anyone whose means of finding information is legal and providing that all such information is duly transmitted in the entirety to the authorities, is any way illegal.
If, for instance, you had means to count how many manholes were potentially open during the ongoing public works at the time in PdL... is that illegal?
The sections you posted cover an awful lot of crimes, such as entering the property/land of a person without permission. And worse still if you are asked to leave and fail to do so promptly.
There are significant restrictions on what one individual can ask of a second individual, and passing that on to the authorities would not absolve you of guilt.
There is a yet higher level of restriction if you do this in a professional capacity.
So I, as a private individual not a professional investigator, would get more lee way than a PI, albeit still with a raft of no-go areas.
I cannot see how investigative journalists (professionals) could make much progress and remain legal in Portugal.
The constitution enshrines the right to discuss any case in Portugal, but judicial secrecy would restrict that to information in the public domain.
I see Mrs Murat's effort has gotten a mention. There was nothing to stop her inviting people to voluntarily pass information to her. That's discussing. If she investigated, she probably broke the law. If she shared any of several items of information with Robert, I would say the police could have made a case against her. Of course, at the time, police energies were directed elsewhere.
Wandering round Luz, counting manholes, and/or discussing known facts would not be problematical.
However, if I were to track down John Hill (easy on a number of fronts) then obviously anything covered in his witness statement is off-limits, as are enquiries about how the OG interview was conducted and which questions were asked. Effectively, this makes him useless as a source of information at this time.