Can I restore the functionality to add/remove rows in quick model after I have edited a process in graphical editor?
The quick model is intended to be useful for fast modeling and creating a basic structure of a diagram. Here you are only able to define one diagram path (without using gateways or sub-processes). After changing the diagram in process editor (especially inserting gateways) the process is not straight-line anymore. Since the Quick Model does not fit this kind of processes, you cannot add or delete any process steps there.
Please use this area to write only internal steps to solve an error.
KBA , BPI-SIG-PM-MOD-EDT , Graphical Editor for SAP Signavio Process Manager , How To