Purchase Order Approval Notification

Purchase Order approval notification always go to Preparer. If the Preparer and  Buyer are two different person, then by default Purchase Order Approval Notification will go to Preparer.
Node 20 of Purchase Order Approval Workflow send this notification. For sending the notification to the Buyer, customization need to be done at this node to add Buyer also in the notification list.
Same place customization need to be done for sending the approval notification to the Original Requester.

Purchase Requisition Approval Notification

Purchase Requisition approval notification always go to Preparer. If the Preparer and Requester are two different person, then by default Purchase Requisition Approval Notification will go to Preparer.
Node 24 of Requisition Approval Workflow send this notification. For sending the notification to the Requester, customization need to be done at this node to add Requester also in the notification list.

Charge Account is not getting defaulted on PO as per Expense Accounts Rule Defined

If you are creating a PO directly (without having Requisition) for indirect purchases, then charge account will not get auto populated in the Distribution as per Expense Accounts Rule. For getting it defaulted as per rule, you need to enter the Requester Name in the Requester field at Distribution level.