Is it possible to resume baking brownies the day after?

It's unlikely to work very well. The outside, which is already cooked, would come up to oven temperature almost immediately. Then you have to wait for the inside to get hot enough to cook, by which time the outside is burnt.

I suggest you might be better off cutting them first (assuming you baked them in a tray) then cooking a little more before serving warm. This means less far for the heat to penetrate, but also gives you the chance to check whether they're just underdone or actually uncooked. That's not easy to test with a skewer, especially with something that's meant to be moist. The difference is that raw or even worse slight warmed brownie mix doesn't keep at room temperature overnight while cooked brownies do (for several days). You'd only be using doneness as a proxy for temperature but it's a good one in this case.

Yeah you can. Two stage cooking isn't that big of a deal. You'll have to pick up around where you left off while accounting for the time it takes the food to heat up. So for brownies that need about 5 more minutes in the oven, get the oven heated to the right temperature while letting your brownies come to room temperature. Then pop them in the oven for 2 minutes. After that take the brownies out and check them for done-ness. If they're not done then just put them back in for a time and check them again.

The biggest issues with two stage cooking is over cooking your food and not cooking it in the right environment so make sure you cook them in the same environment as you did the first time and take extra care not to over cook anything.