LWR and Flows – Compatibility Issues Between LWR and Visual Flows

If you need to make a picklist value dynamic based on the field value of another record, use the dynamic field variable in the choice.

Add this choice in the picklist and select it as a default.

Now, since there isn't a 'Validate Input' option for picklists, any validation for picklist values can be added in the Decision after the screen. The user can be redirected to an error screen if the selected picklist value doesn't meet the required criteria.

In the example below, a user is redirected to an error screen if he selects the picklist option 'Please Select a Value' and clicks on Next. Also, in the error screen, the Next button is removed. This makes the user select a value other than 'Please Select a Value' in order to proceed with the next steps in the flow.

I think that the best approach would be first doing all the sums, and the formatting as you like

Object.entries(
   input
      .reduce(
         (prev, current) => ({...prev, [current.StockKeepingUnit]: (prev[current.StockKeepingUnit] || 0) + current.Quantity}),
         {}
      )
).map(([StockKeepingUnit, Quantity]) => ({StockKeepingUnit, Quantity}))

For your second question, it would be very similar to the first answer, but instead of adding the values, you should add them to lists, and then reduce the items again in order to calculate the average

Object.entries(
   input
      .reduce(
         (prev, current) => ({...prev, [current.StockKeepingUnit]: [...(prev[current.StockKeepingUnit] || []), current.Quantity]}),
         {}
      )
)
    .map(([StockKeepingUnit, Quantity]) => ({StockKeepingUnit, Average: Quantity.reduce((a, b) => a + b, 0) / Quantity.length}))