"Marcus and Livia are coming from the city now."
Translation:Marcus et Livia nunc ab urbe veniunt.
My understanding is that in can take either ablative or accusative, but the meaning is different. In urbe (ablative) means "in the city", as in "The student is studying in the city." In urbem (accusative) means "into the city", as in "The soldiers are marching into the city."
It's often in first or second position as a discourse marker but you'll have to be patient and keep an eye on how Romans used that particle. E.g., Cicero, Ep. 1.5b,2 nunc id speramus idque molimur, ut rex... Cf. Davide Ricca, "Adverbs" in New Perspectives on Historical Latin Syntax 2: Constinuent Syntax... (De Gruyter Mouton, 2010): 109-189.
I don't think you would ever use esse + present participle like that. It looks very strange... You are actually emphasizing that Marcus and Livia are present, not emphasizing the "coming" word. You sentence reads something like: Now here are Marcus and Livia, who come to the city.
Or possibly: Marcus and Livia are coming to the city, and now here they are.
Latin often uses present participle phrases when in English we would say: person does -participle- and then -verb-. Or while person does -participle-, he -verb-.
"Marcus et Livia nunc ab urbe veniunt." was given to me as a suggested correction, so, your sentence, with a different word order is right. (There's maybe a preferred place for the time adverb "nunc", but I don't know)
"ing" is really an ongoing action, so you can use each time "nunc". (but not mandatory when the "nunc" is not there: it's still right to translate "Marcus and Livia ab urbe veniunt" with either the progressive -ing form, or with the simple present.)
Only the "nunc" made it mandatory to use the -ing form.
Here's a quick overview of nunc and iam: https://latinforaddicts.wordpress.com/tag/iam-vs-nunc/ For a deeper dive with secondary literature, see See Caroline Kroon and Rodie Risselada, “Phasality, Polarity, Focality: A Feature Analysis of the Latin Particle IAM” Belgian Journal of Linguistics 16 (2002) 63–78, here 73. iam sometimes has a focalizing function that lends vividness to a narrative and calls attention to a particular point.