RE: Item separation by purchaser
There is e.g. a report "Vendor/Item Purchases" (313). There you can add a filter by Purchaser and get the results, filtered by certain purchaser.
If you are familiar with NAV tables, some information you can get directly from the Item Ledger Entry table (32). There is a Document No. field which relates to the same field in the Purch. Rcpt. Header table. Purch. Rcpt. Header table contains the Purchaser code, so for each positive adjustment of the item you can try to find a purchaser code through this relation.
In Value Entry table (where the items amount is basically registered for items transactions) there is User ID field. This would give you the information about the user who made a posting of the item if your filter by "positive adjustment" type and Item Ledger Entry Quantity field not zero (otherwise here you will also have transaction of e.g. items charges allocation to already existing purchases, which you most likely do not need). However the person who posted the document is not necessarily the purchaser, defined in the Purchase Order.
I do not recall quickly a ready out-of-the-box report that would show the purchases made by each purchaser... Perhaps, other members will recall that.