How do you use tomato curry sauce?

I would use it (plain) in place of tomato sauce for a flavorful twist on old favorites. For example, as a pizza sauce for a Mediterranean/veggie pizza; thinned and used as pasta sauce; maybe even as a sandwich spread.

ktr, Ms. Basu wrote an article about this very recipe, with links to a number of different recipes.
https://food52.com/blog/13865-curry-on-how-to-use-indian-curry-sauce-in-5-dinners
She has a link to exactly what you're thinking about making, chana saag: http://www.mallikabasu.com/2014/11/27/chana-saag/

I'm about to make a batch of that sauce within the next few days, for that very purpose - adapting it for a variety of meals over the next month or so. The recipe makes enough for freezing smaller portions to then use as a building block / "weeknight dinner starter", something I'm always on the prowl to find. (I've even started a recipe collection for such helper components.)

And, just by coincidence, I actually bought spinach at the farmers' market, and garbanzos at my desi grocer, for making that saag. So the old saying about brilliant minds thinking alike rings rather true, I would say. ;o)

For the latter question, you ask the question from the recipe's page. For example, go to:

https://food52.com/recipes/35952-tomato-curry-sauce

scroll down to "Comments" then click the adjacent "Questions" link. From there, you should see a graphic that says "Have a question about this recipe? Ask Food52 Hotline." Clicking that should automatically include a photo linking back to the recipe in question in your Hotline inquiry.

As for your first question, I think it really depends on what you eat and look at this sauce as a condiment. I eat a lot of veggies, so having a small amount of this on the plate would make a nice occasional complement. That said, I personally find curry to be a rather dominant flavor, so I would use this sauce sparingly.

Per the recipe author Mallika's head note, this is a base sauce that can be tweaked later with additional spices, much like a classic plain Italian tomato sauce is just a base sauce. Theoretically, most of the dishes that call for tomato sauce could conceivably be suitable for the tomato curry sauce. It really depends on your taste buds, those of your dinner table guests, and your personal vision of what you want to serve.

Good luck.