Before going in and trying to flatten or restore anything, try to get as clean a scan as possible of it just as it is. Depending on your scanner configuration, you can likely flatten it under glass (if transmissive) or under a piece of white board (if reflective) to minimize any warping provided the original is not too brittle.

There is a good chance that will give you a good enough scan to use it as is with minor retouching. Especially if your example is typical of the condition. If not, at least you will have preserved a version you can work with if everything goes sideways.

Beyond that, it's very difficult to say what your specific transparencies can handle. Any mechanical, chemical, or thermal solution is as likely to damage the medium or the information as it is to improve them.

When I've done restoration work, I've avoided modifying the original piece whenever possible. And most methods of removing curl from film or prints that are too brittle to just flatten involves, at some point, slowly and carefully modifying the moisture content to relax the medium. Obviously that could damage dry erase marker very easily.

All that said, you'll have to experiment, but do so slowly and conservatively if you have to. If you decide you need to work on them, perhaps start by placing the transparencies on an actual overhead projector (they heat up nicely, but the transparencies are designed to handle the amount of heat they produce.)