1. Yes, DBA looks at PO-H first but you need to have pricing in PO-H for every vendor you may purchase something from to have it there to pull. IS Tech has an option that PO-H can be updated by vendor with each PO receipt if you turn it on in SD-Q so you can automatically populate the PO-H database.
2. Who knows?

But that is why we would make it a default setting rather than change it for everybody. I can see it if the standard cost is based on purchasing production quantities but occasionally you pay a premium for an expedite or something, you don't want that premium price pulling in as Last Cost on the roduction quantity PO.
3. The standard costs for purchased parts can be entered by clicking the Standard Cost button in IN-B or in IN-L-A.