Q1. Your buyer should be updating the price details in the purchasing document as this is your contract between the vendor and you. You will sent the updated scheduling agreement contract to your vendor to confirm your pricing agreement. The update of the price in the purchasing document will update the info record purchase order price history. Changing a price in a scheduling agreement will not update the condition record in the info record.
Q2. By entering the price in the scheduling agreement you make sure that the price in your purchasing document is correct
Q3. Your goods receipt is posted against a purchasing document not against an info record.
As been mentioned before the info record is a master data record that is used during the creation of your purchasing document. Once the purchasing document is created this data become independent and must be maintained independent from the info record.
The same goes with things like the planned delivery time of the goods receipt processing time. These get copied at the time of creation of the scheduling agreement but then these become independent data. If you change your MMR then these changes are not reflected in the scheduling agreement.