Lets just say you are fairly safe if you stick to the list in 119.1 (e). That's why so many aspiring airline pilots go the CFI route as it's specific in the regs and airlines understand what being a CFI is about.

I'm not the FAA. But I believe so. The FAA's primary concern is safety. And I believe, without looking it up, under Part 135 you need a min of 1200 TT, with some additional sub-times to fly passengers as PIC. If you notice under the list in Part 119.1(e) (Part 119 breaks out which operations fall under what regs. The various parts - 125, 121, 135 - detail whats required in those operations), there are no passenger carrying operations beyond 25 miles and even some of those have restrictions. That's why CFI's in this situation will give "instruction" to their new "student" to try to avoid problems here. However, the FAA is not dumb. And if they find out and are concerned enough, they will look for a legit reason for the instuction, ie, an aircraft checkout or flight review or IFR currency, etc.

There are various levels of 135 certificates. The least complex is a single-pilot 135. Most small FBO's will have one so they can fly charter in one of their more advance single or mulit-engine airplanes. The certificate will specifically list the pilot (usually just one or two) and the airplane. The next time you're at your FBO ask if they have a single-pilot 135 and if you could see the actual certificate. Single-pilot 135 certificates are easier because you don't need quite all of the various manuals, training programs, op specs, etc. required of larger part 135 and part 121 operations. You still need some of them, however.