Hi Mahesh,
what I can say is it could have been much easier if it was from Studio 
.
I am not sure how the integration happens here. If the data is sent trough a single service user having access to cloud?, probably then you can do it the way you mentioned. If that is not the case it would be good if you can at least make out the system which is making changes.
You can get the current users identity UUID using
Context.GetCurrentIdentityUUID(); and build your logic.
But this is not as good as disabling the fields on UI because it will confuse user when He/She clicks on the fields as they turn in to edit mode and you throw an error message when He/She tries to save the changes saying you are not authorized.
Anyways I think you have visited all the options and nothing is left, Meanwhile check with SAP if possible, is there any alternative way to disable fields for your use case.
Regards
Vinod
