I think the guideline answers are providing extra information for our understanding. I don’t think they are expecting full paragraphs. I usually write only a couple sentences. When reading the answers, I see if 1) I get the key points they are looking for and 2) if there are any keys or explanations that I feel they’d be looking for. I don’t expect to write them full on pages of information. Doing that would cause you to run out of time for sure.
Be very sharp and to the point. If the question says 3 minutes, make sure that the answer you’re about to write won’t take more than 3 minutes. Short, sharp sentences should do you fine, depending on the command word used in the question.
These aren’t essay questions, so don’t write essays. Bullet form answers should be fine as long as you have got the point across. I’d strongly suggest that you practice writing short answers. I didn’t do so in my first attempt and got really overwhelmed as I couldn’t manage my time.
I have not gotten guidance, but I was a University teacher and/or TA for 6 years. I graded it how I would have graded it back then. And I’m the case you mentioned, yeah I give myself 1 point for the right choice and allocate remaining points among the reasons. (And I 0 myself if I make the wrong initial choice). And for the calculations, I tend to grade them like Kaplan suggests, by giving points to certain milestones in the problem. I also tend to 0 beyond the point of a mistake, as an extra layer of conservatism in my grading.