First, a note about my perspective here. I am in Zone 3 and as a junior I had very old school trainer. The instruction was that you never do anything that the judge doesn’t tell you to do- so if the judge says “walk and reverse,” you reverse and halt, because Simon didn’t say walk on. Etc. I posted something about this a few years ago after participating in an equitation class that included riders of multiple generations, some of whom reversed and halted and some of whom reversed and walked. I will say that the ONLY COTHers who agreed with reverse and halt in that scenario were ALSO in my state and ALSO rode with instructors from a similar school. I am an announcer and asked a fair number of judges about this- at least as of 5 years ago, the people I asked were evenly split and those who did say “yes, you halt” also noted “it’s a Maryland thing” or “it’s a Virginia thing.” It’s possible that there are also some regional differences that may apply to what people in your area would commonly do if given an instruction like the ones you posit.
So, for #1, I would say you hope that doesn’t happen 
but I would remain in half seat, as I had not been told to sit the saddle.
In the equitation section of the USEF rulebook, under the list of tests, it’s specified that turn on the forehand may be called for from walk or halt. Turn on the haunches must be done from the walk. By the way, for what it’s worth, in my region and zone half-seat at walk or trot is a common direction in all levels of IHSA and IEA class.
The rulebook does not specify in which gait to exit from the turn. I would exit at the pace at which I began. So, if the instruction were “walk, show a half turn on the forehand,” I would walk, show a half turn on the forehand, and walk on. If it were to halt and turn, I would halt, turn, and, after having performed the turn, halt and await further instruction. If it were to halt and reverse, no method specified, the only correct maneuver is to reverse with a turn on the forehand.
In reality, in IEA classes, my experience as the announcer at quite a few regional and zone finals is that your judge will be very specific about what they want, and if the announcer does not relay that exactly, the announcer will be corrected. I think it is unlikely that your riders will be given any imprecise directions.
I also think it is unlikely that your riders will be asked to show a turn on the forehand unless the show host can guarantee that every horse in the draw for the open division is schooled to it. I say this with love towards the many wonderful school horses in our region and zone… I don’t think that would happen in my neck of the woods. (Again, as an announcer, I have had the judge ask me- “Any idea if these horses counter canter?” in a particularly strong open flat class. I did have an idea… The judge selected another test.)
