Just my two cents:
Don't use SAP BPM if you can use Business Workflow!!!
I'm using SAP BPM for almost 4 years now, and it's still a pain in the ***...
It's still very buggy + unstable and developing with NWDS is also very uncomfortable.
With ABAP stack you can easily use standard Eclipse IDE SAP Web IDE.
Additionally version management/transport mechanism with standard CMS is also much better than if you have to set up a NWDI environment.
So my recommendation:
If you can stay on the ABAP side, stay there...
Regarding FIORI I have no clue, but I guess you can also build SAPUI5 applications for approvals without activating the FIORI stuff. The only downside I see is that you have to build up knowledge regarding .
Or you use WD ABAP if the approval will only be done on Desktop PCs (it now also supports Chrome and Firefox very well).