Hi,
Konstantin is right. The points mentioned by him should be taken care.
1. You are not allowed to change SC created from purchase requisition. No completion workflow. - Yes correct
2. Changes of purchase requisition are also sent to SRM and SC updated according to changes. What should happens with Workflow? Restarted? - Yes the WF will restart.
3. What happens if approver rejects shopping cart (at least 1 item)? It goes to creator, i.e. technical PI user. You are not able to login with system user to accept the changes.
You need to develop a custom report that checks the SC created by RFC user and accept the rejection using FMs related to WF and schedule it as background job.
4. If SC rejection is accepted - how the purchase requisition to be updated? Which status to be set and how? - You can set the status of PR item to deleted in DOC_SAVE BADI using BAPI_RQUISITION_CHANGE checking the status of SC and type as 'ER'
So basically if you want to use approval for PRs in SRM a lot of development is required apart from the things mentioned in the document but it is possible.
Thanks,
Anubhav