"Ahora estoy comiendo" & "Estoy comiendo ahora" are correct. It depends on what part of Ámerica you are, in México and Colombia most of the people use an active form while in some parts like Sudamerica they use the passive form.
Now, in the real world you will hear this expressions when the person is annoyed while eating and is asked to interrupt their meal and do something else.
Pay attention to the the tone used, you will hear the stressing in the words "ahora" and "comiendo"
If this is a something you want to let know you can use a simple, "Estoy comiendo"
-¡Carlos! te pedí que sacaras la basura, ¡ahí viene el camión!-
- no puedo, ¡ahora estoy comiendo!
-te llaman del trabajo, quieren que vayas a arreglar un asunto.
-¿de veras? Diles que estoy comiendo ahora, ¡que se esperen!-
-Hola Bryan, ¿pasamos por ti para ir al museo?-
Estoy comiendo, ¿nos vemos en 1 hora?
Some customs notes:
Ámerica is the continent, they call the country: United States, so you will find some tourists aboard when they are asked where are you coming from? stating: America (so people will know from which continent) and then mention the country Chile, el
Salvador, Colombia, etc
I know is a lot of controversy around this but if you want to settle it; just look at the rockets from NASA and the Apollo missions, they are branded United States, not America.
Of course languages are a live entity so you can foretold changes and adjust to local customs.
In the Hispanic cultures in the urban areas you "desayunas" from 6-8 (in the countryside 4-6am "Tomas tu café" is a small meal that usually consist of coffee and sweet or savory bread and have their breakfast around 8-10 am)
"Almuerzas" around noon both in urban and countryside areas
You will have your main meal from 2-4pm (soup, entree and cheese with beans, usually tortillas are part of this meal)
And from here, changes between areas, some regions will have merienda around 6-7pm and cena 9-10 in the urban areas while in the fields you will have only your cena right at 7 pm.
Hablar y escribir bien, reader' digest México ISBN 968-28-0195-8