The recipe has only been tested with rice. Quinoa would be fine though with a little adjustment. It would need to go in uncooked as the rice does. BUT, because its absorbency is different to rice and its not as starchy, you will need to adjust the amount of total liquid in the soup. I'd cut it by a quarter. Then if it seems too thick once the quinoa has cooked just boil a kettle and add some boiling water to thin it a bit. Hope that helps!

Thanks for the informative response! Made this AGAIN today for St. Patrick's Day!

Glad you enjoyed it! I haven't made this in the Instant Pot with dried lentils. Only cooked lentils. You will definitely need more liquid if you use dried ones or it will be way too thick and you'll trigger the burn error.
Remember if you are putting in dried lentils you'll need to half the amount as the quantity given is for cooked. If you're adding 1 cup of dried lentils I'd probably add an extra 2 cups of water. Hopefully, that should be enough to not trigger the burn error. I'd guess about the same in a slow cooker too. Hope that helps!

I don't see why not. Just don't use red lentils because they tend to lose their form.
As for brown rice in the IP, I haven't tested it that way so can't say for sure, but I'd go for 15 mins high pressure and leave it to naturally release for 10 mins before releasing the rest of the pressure. It should be done. In the unlikely event it isn't, just switch it to saute for 5 mins or so to finish off but I don't think you'll need to.
Also I don't recommend omitting the sugar. It's important for balancing the acidity in tomato-based recipes.
Hope that helps!