Hello,
I’m wondering if it is possible to compile a project that uses the SDK for 2015 when the project is not going to be used in a Rhino Plugin
No. You can use the OpenNURBS library outside of rhino, but not the rhino sdk. OpenNURBS contains routines to read and write the rhino file format, and therefore contains the entire geometry data model of rhino. It also has basic geometry functions, but more advanced algorithms are proprietary to rhino (intersections, trimming, Boolean operations, etc.)