KH,
RMMRP000 is used during background MRP runs.
RMMPS000 is used during background MPS runs.
MPS usually only plans materials that have an MRP Type of M*, although there is a switch that you can set to also plan MRP materials as well. RMMPS000 will always explode demand down the entire BOM for all the materials in the BOM that are categorized as MPS. RMMRP000 will in general always explode the BOM all the way down to the highest Low level code (the 'end' of the BOM) for ALL MRP relevant materials.
For your Q3, put your cursor on the 'Create Purchase Req' field, and press F1. Actually, this works on most fields in SAP software, try it.
Expert Jürgen, who has considerably more patience than I do, has kindly indicated the help doc for you, which explains the MAP calculation. I am still baffled why anyone would want to manually calculate MAP for a material that is Priced at Standard.
Best Regards,
DB49