Hello,
You should use customer metadata to implement this requirement.
Use SM30 and configure in view/table /SAPSRM/V_MDA_HC for action EDIT and Obj type BUS2121
You will need to create Dyn Customer class/ method to Disable the Edit button by reading the current workflow data of the cart.
To read the workflow process data you can use /SAPSRM/CL_PDO_DO_APV_EXT method /SAPSRM/IF_PDO_DO_APV_EXT~GET_PROCESS_INFO
You can refer to existing standard code on how to instantiate this class and how to loop through the workflow process info table.
Regards,
Sushil.