Sugar – How much jaggery can replace 1 tablespoon of white sugar

A simple alternative would be to make a ganache flavored with Frangelico or another hazelnut liqueur. This has the advantage of having much less sugar, and probably better overall flavor, than an even-more-sweetened Nutella, assuming you start with a reasonably dark chocolate. All you do is boil cream, and pour it over a similar quantity by weight of chopped chocolate, into which you've added a healthy splash of hazelnut liqueur (or the liqueur can go into the cream as you bring it to a boil; either way works in my experience). Then stir vigorously until all the chocolate is smoothed out. You can pour it over while the mixture is still moderately warm and refrigerate it with the cheesecake. It will not be Nutella, but from experience it will be good, and evoke the same basic flavors.

Alternatively, most cheesecakes I am familiar with aren't topped with an icing, but a mixture of say cream or sour cream and other ingredients, sometimes added in the last few minutes of baking. These usually spread out reasonably well. You could consider softening the nutella in the microwave and mixing with a bit of cream, which will be a sort of less heavy ganache.

From my experience coloring regular sugar with ordinary food coloring, drying it, and then crushing it in a mortar yields good results. Be careful to not use too much food coloring though, or it will never dry properly! I haven't tried to see if cornstarch might save it, but as long as you do the drying properly and don't overdo the amount of color it shouldn't be necessary.

As for the powdered food coloring option you suggested, I found a source that said they don't have to be wet to take effect, but it doesn't say anything about whether the eaters will be colored =)