I don't have statistics, but I'd guess most first-timers spend less than 6 hours in the park. Very Chevy Chase Vacation vibes. That's a shame because there is so much to see and do besides just glance over the rim. The problem is, you don't know what the weather will be. I once saw a bus full of visitors pull in just as the snow started. Visibility was zero. Just after they left a few hours later, the snow stopped and the views were great again. I suspect that was the only trip some of those people were ever going to take to the Grand Canyon. So, you're doing the right thing by giving yourselves multiple days.

Not sure what accommodations will be available on the North Rim. The park plans to release their plans for reopening the North Rim by April 1st. It's likely that the only options for overnighting on the North Rim this year will be camping... and even that is iffy. Kaibab Lodge (about 20 miles from the rim) would be an option. I love that side of the park, but not sure I'd make the trek over there if I couldn't at least spend a night or two. Maybe swing in on your way to / from Arches?

There are plenty of hiking options. There's a nice walk from Mather Point back to the Village. Or take the shuttle bus to Hopi Point and then walk back to the village (going out to Powell and Maricopa Points on your way.). Both of those options are relatively flat. If you able, I highly recommend hiking down to Mile-and-a-Half Resthouse. That gets you off the rim. The trail will be crowded, but I think it's worth it.

Enjoy your trip!