I am being handed a Indesign (.indd) file from which I extract out .idml file using Indesign App. Can this & other way conversion be done through Adobe's SDK or any online api without the need to have the app installed. Didn't see any reference for this in the Adobe's SDK documentation. Also, I need to preview the Indesign document on a web-browser, I am currently able to do so by uploading the .indd file in Adobe CC. Are there any other alternatives for online preview on Indesign files?
Sooner or later you will need the InDesign engine to get the rendering of the file. There is a java lib that let you play with idml files (idmlib I guess) but unless I am wrong it won't let you get a rendering out of InDesign.
If you are into a web based solution you may consider InDesign Server to get the rendering of the file but as I said you cannot render indesign files outside of InDesign.