am assuming this is a concrete structure, when shifting service toilet to utility area, and if there are apartments above, then running extra lengths of supply/drainage lines & introducing a structural sink slab (if this is concrete) under the new location can be charged extra.. it will be easier to divide the 3rd toilet. all services will be already available. only, do check on supply & drain pipes capacity as load is increased on the same pipes which were serving one toilet earlier..

Temporarily you can lay a thin bead of caulking it will come off both the fridge and counter cleanly. Not seeing the issue and possible space it's difficult to advise you further. Possible to take a picture?

You can create partition between the two spaces by smartly positioning the furniture.

It Depends on the size of the cut out and how is it connected to the other parts of the building. For example- 1. If it is connected to the kitchen and living room and has sufficient space can be used as dining with double height and skylight. 2. We can hang a chandelier that kind of shines or be something visually appealing in natural light 3. for function based we can make it a electrical room or a small storage along with skylight P.S. there are numerous options but we need to find a suitable function of it first and we also need to see if it is either designed for other thing rather than just being a sky light.