gistfile1.rb

# The fundamantal design opinion is that a Stock Return has many Returned Items 
# Each Returned Item effectively has one of 3 actions associated with it return, exchange OR replace
# Any of these action can involve a refund, although the most common is that a return involves a refund
# You may wish to refund a replacement Or exchange to maintain customer relations
#
# Once all Returned Items and their actions have been set then you can commence to attempt to process the Stock Return
# Processing the Stock Return will automate the Order Adjustments, Addiitonal Shipments and any Refunds / Payments
# It should be exstensible so that developers can easily implement their own behaviour here
#
# It could be that a developer can extend this to Create New Orders instead of Additional Shipments under certain scenarios:
# When the total of the exchanged items exceed the balance on the Order, it might be preferable to require a new payment, hence new Order

class StockReturn    
  # return_cost
  # handling_cost
  # timestamps
  has_many :returned_items
  has_many :exchange_items
  
  before_transistion_to :complete, :handle_return
  
  # Validate processable?
  def processable?
    Spree::StockReturnHandler.factory(self).processable?
  end

  def handle_return
     Spree::StockReturnHandler.factory(self).process!
  end
end

# A Returned Item has 3 actions: return, replace or exchange
# You can refund (using refund_amount not null and > 0) with ANY of the actions above
#
# Attributes:
# - reason: provided by customer (wrong_size, not_suitable, not_as_described, arrived_late, other, etc)
# - reason_other : string
# - refund_amount: allow null, null is unknown and therefore unprocessable, must be set to 0 if no refund provided
# NOTE: No qty for line item, each one has a separate record
#
# Potentially:
# - condition: provided by receiver (faulty, damaged, good)
# - restock: boolean (null is unknown and therefore unprocessable)
class ReturnedItem
  enum action: [:return, :replace, :exchange]
  enum reason: REASON_CODES

  belongs_to :stock_return
  belongs_to :line_item
	
  # If certain Reason Codes provided then we can auto set refund amount
  before_create :auto_set_refund_amount
  
  def refund?
    refund_amount.present? && refund_amount > 0
  end
end

class ExchangeItem
  # price (defaults to line_item current price but if set here is overriden)
  belongs_to :stock_return
  belongs_to :line_item
end

class StockReturnHandler
  def initialize(stock_return)
    @stock_return
  end

  def processable?
    # These methods add errors to the Stock Return object
    refund_due_within_processable_boundary?
    exchange_items_shipable?
    replacement_items_shipable?
    @stock_return.valid?
  end
	
  def perform
     return false unless processable?
	  
     adjust_order_for_return_cost
     adjust_order_for_handling_fee
     create_shipments_for_exchanged_and_replacement_items
     process_balance #(refund or charge additional)
     
     true
  end
end

You said you would like the ReturnedItems to not have a quantity, and be a single record per unit quantity. That is what InventoryUnits are used for in terms of mapping LineItems to a Shipment. Following that naming convention it would make sense it using the naming as ReturnedInventoryUnit and ExchangedInventoryUnit as the way to track that your shipped InventoryUnit came back.

https://github.com/spree/spree/blob/master/core/app/models/spree/inventory_unit.rb