Rijksmuseum questions

“1) How strict are they with entrance times? If I buy a 9am ticket, and I get delayed, will I still be able to enter, say at 10am, or later?”

The rule is that you have to enter at the time of your chosen time. If people start turning up at random times there is no point in time slots in the first place. If you ask nicely, you may be allowed in at another time, but this is at the discretion of the staff member checking the tickets.

“2) Can I leave the museum grounds and be able to reenter later that same day? I thought I read that you could leave & reenter the museum, but I can't tell if they mean the whole complex, or just the museum”

No, when you exit the museum your exit is final and you’re not allowed to re-enter.

“3) How good is the restaurant? Typically, I find that museums have ok food-something to tide you over. But I have also been surprised that some museums have extraordinary restaurants that people make a point of eating at.”

There are 2 restaurants; one inside the museum and one just outside the museum. The restaurant inside the museum serves soups, salads, sandwiches etc. The restaurant just outside the museum called Rijks is a fine dining restaurant with 1 Michelin star. That restaurant is one of my favorite fine dining restaurants in Amsterdam.

re #1, on the museum's ticket info page it says this:

I’m running late, and I’m not going to make it to the museum in time
for my start time. What can I do?

Unfortunately, we cannot help you in this case.

We ate at the restaurant inside the museum last year. It has a more extensive menu than just a coffee and bakery type place. It was fine, good service. And quite crowded - I think we entered the museum at the earliest slot on a Sunday morning and an hour later, the cafe was nearly full.

When I was there spring, 2024, they didn't seem to care about the time on the tickets. However, the museum wasn't sold out for those times.
The museum was uncomfortably crowded in the afternoon, and that really impacted my enjoyment. As much as I hate early mornings, I would arrive when it opens. I have also read that later in the day, or the one evening, are also less crowded.
The museum places the works of high interest in the same general great hall, so everyone goes there.
Have a great trip!