Do Federal Regulations really require compliance with all crewmember instructions?
I found an overview from a lawyer on what "interfering" actually means. The short version is that there's no regulation or law that says "you must obey all instructions". Instead, they say "you must not interfere" and that's interpreted very broadly.

She based the article on one regulation and one law:

14 CFR 121.580 (see also 91.11 and 135.120)


  No person may assault, threaten, intimidate, or interfere with a crewmember in the performance of the crewmember's duties aboard an aircraft being operated under this part.


49 USC 46504


  An individual on an aircraft in the special aircraft jurisdiction of the United States who, by assaulting or intimidating a flight crew member or flight attendant of the aircraft, interferes with the performance of the duties of the member or attendant or lessens the ability of the member or attendant to perform those duties, or attempts or conspires to do such an act, shall be fined under title 18, imprisoned for not more than 20 years, or both. However, if a dangerous weapon is used in assaulting or intimidating the member or attendant, the individual shall be imprisoned for any term of years or for life.


The lawyer's analysis is that not following an instruction would be a violation of the regulations (civil penalty). If you go further and physically interfere with a crewmember that would be assault (criminal penalty).

When it comes to instructions specifically, she says that "interference" can be interpreted very broadly (emphasis mine):


  Almost any offensive or disruptive behavior that distracts the crew
  can be considered interference, such as:
  
  [...]  
  
  
  disobeying repeated requests to sit down, return to your seat, or turn off an electronic device
  


Her basic advice is to follow all instructions if possible and never touch a crewmember:


  Any time you disobey a crewmember’s instructions, you run the risk of violating federal law. But civil penalties and criminal prosecutions usually result only when passengers repeatedly ignore, argue with, or disobey flight attendants; or when they act out in a way that is dangerous.


You can see the FAA's enforcement statistics on this, by the way. Interestingly, it seems that the number of cases fell sharply in 2005 and has been trending down since then.