Have you attended a course run by the organization at dhamma.org? If so, where, and did you feel physically and mentally safe while there?

Yes. I've taken 5 (or 6?) courses, all at the Dhamma Dhara center in Massachusetts. I did feel physically and mentally safe there. I'll try to address the red flags you mentioned.

First it should be noted that the centers are somewhat independent, even though the course is quite standardized. They are run on volunteer donations and time, and as a result the set of people working there changes often. Most just come to volunteer for a single 10-day course. Therefore your experience will be partly dependent on the volunteer pool for that center. Some geographical locations will be probably better than others.

Now to your red flags:

amount of sleep

The course schedule does indeed indicate that you should get 6.5 hours sleep, but you don't have to. I'm fairly sure a majority of students skip the first meditation session (from 4:30am to 6:30am) and simply get up for breakfast at 6:30am, allowing for 8.5 hours of sleep. Personally I didn't have much trouble getting up for the early session.

lack of real engagement with teachers

This could be a problem. The teachers seem to mostly parrot Goenka's instructions, even though they have extensive meditation experience. It's unfortunate. I didn't feel the need to ask many questions during my courses so it wasn't a major issue for me. More personalized instruction would be better and could allow for faster progress.

pressure not to leave the course

As I've never tried I can't speak to this personally. At the start of the course they do encourage you not to leave until the end. I've volunteered for some courses at the Massachusetts center and I've observed the procedure there. If you want to leave a volunteer will first ask you about the problem you're having. If they can address it they will, otherwise they'll ask you to talk to the teacher about it. I think the teacher usually encourages you to stay for one more session or something, since the desire to leave could be the result of a transient emotional storm. If they still want to leave, volunteers will help them get their stuff (and a taxi if need be). Again, your experience will be partly determined by the people who happen to be there.

In my experience the dhamma.org courses provided a positive and supportive environment. You will have to endure some (but not too much) dogma/garbage, which will probably also be the case in courses elsewhere.