Sadly, this entire discussion makes me wonder about so many grey area (hypothetical) cases:
- A student-owner needs maintenance on the a/c and asks the CFI to fly along
- A student who wants to take up his wife to see if she will enjoy flying before buying an airplane
- A new IR pilot wants a CFI to come along in his first foray into IMC
- The newly minted PP who wants to take all his freinds for a ride and does 10 short hops in one day (and doesn't ask for reimbursement)
I think you can go overboard with the concerns. The primary rule I think the FAA uses in these cases is the "quacks like a duck" principle, and the test I use for myself is the "How would you explain it to the FAA after the incident?" Neither requires an in depth legal analysis, just a little sense.
My problem with the IFR cross country for a 1-hour student to pick up someone is that I just don't see the instructional purpose for the trip. It "quacks" like a paid flight to pick up a passenger. I could certainly be way off base, but I don't see the FAA buying, "Well, I really thought that this new student who can barely hold altitude or heading visually or communicate with ATC would learn a lot from an IFR flight that is heavy on flight solely by reference to instruments, using the advanced equipment in the airplane, intercepting and tracking navaids and constant communication with ATC."
Make the "student" in the IFR cross country scenario to pick up someone the new IR pilot in your third scenario - "I want to pick up someone and it's IMC. I'm a little nervous about it so I'd like an instructor with me" - and I barely see a problem at all. Make it somewhere between those two extremes, and we will definitely get into gray territory where a small change in the facts could make a big difference in the answer.
In fact, without more details, the only one of the 4 scenarios you posted I'm not comfortable with on the short set of facts is the first one. Is the student a student pilot? If so, no problem. If the student is a private pilot, is the CFI there purely as a friend/passenger or is there some reason the pilot wants a CFI there? Going a step further, my flight with a friend does not become an instructional flight just because I'm a CFI, even if I have given my friend instruction other times and even if I can't keep my big mouth shut and end up doing a bit of incidental instruction during our flights together.
Your second scenario is a really good one. Some significant others are hesitant about going up with a new pilot and a CFI's presence the first time can make a difference. "My husband is terrified of flying. He doesn't want a lesson himself, but I think it would really help to have an instructor on board the first time I take him up" is, to me, anyway, a completely legitimate instructional purpose. In fact, although not necessarily in the purchase context, I generally recommend to my students that they bring a significant other with them during a lesson; I even ask who else in the family wants to come along on a discovery flight.
Subjective? Well sorta. I think what we're talking about is the measurement of subjective intent by a quasi-objective reasonableness standard. No, it's not comfortable. Everyone would like a nice black and white answer where you look at very simple things (like whether or not signed a logbook) and that would be the end of the story. The area between private and commercial privileges and Part 91 and 135 ops just isn't that way.
I'm sure Bruce remembers when FAA Legal said that compassion flights like Angel Flight and Lifeline Pilots were ops requiring commercial pilots and Part 135 operating certificates.
