You are being tested on your ability to maintain and comply with the standards for the maneuver you are executing. That doesn't mean perfection, but it does mean within standards. If the DPE is flying as a student, then they are usually looking for you to point out errors, you can't be responsible for their actions while they are flying, except to take the controls during safety of flight scenarios and when a negative transfer of learning is occurring from them doing a maneuver wrong. Sometimes they are looking for you to critique their performance after they accomplish a maneuver and tell them whether it was within standards or not.

The simply way I interpret this is: If the candidate is on the controls, they must comply with the applicable standards. If the DPE is on the controls and evaluating the CFI's performance, the CFI is responsible to demonstrate their instructional knowledge and maintain safety of flight.

Power off 180s have always been a little bit in the grey, since once you "start", you either complete it successfully or not. If you are doing this in real life, you have no "go-around", unlike a short-field landing. I've known a lot of DPEs to give some leeway on this and honestly if something comes in and interferes like a downdraft then you should get another attempt, but that can be a touchy area, was it your planning, or a downdraft? I see lots of people flying "best glide speed" on the downwind past their point, which makes little sense to me. So you are going to fly best glide, maximizing the distance in the opposite direction to where you want to go, often with a tailwind, then you are going to turn INTO the wind and now you are forced to maximize your distance on final. IME, going to "best glide" is not warranted unless you are on final and need to extend your distance there. I tried to teach these as a "box", you keep yourself in the box where you have options, ways you can get lower or higher on the approach path, vs. flying best glide early on and screwing yourself later. I got really far into these and came up with specific criteria, amounts of wind, distances and altitudes and a formula where to make the turns so it would be repeatable and reliable for various conditions. The reason I got so analytical with them is the "catch-22" of the maneuver, technically it has to work the first time. If my students got a "2nd attempt", all the better and in the rare case they didn't nail it the first time I'd given them enough tools they would be able to analyze why it didn't work. IME, private and commercial students are not very good at "guessing" and they don't have the thousands of hours that are necessary to operate much more on feel, so you have to provide a good structured basis for their decisions, on which they'll develop those higher skills.

Not a direct answer, but if something unforeseen interferes with the maneuver, it may be repeated. Sometimes it's hard to get a tower to give the spacing, sometimes an airplane hasn't cleared the runway, etc. If it's your planning on a power-off 180, probably not. Doesn't mean you shouldn't go around, continuing and landing when you know it won't be in standards would be even worse obviously, but due to the nature of this maneuver there may not be a "second chance". Same thing with an engine failure to emergency approach to landing task, you don't really get a "go-around" on that one either.