Hi David,
As far as point 1 and 2 goes it can be achieved using standard config. You just need to define a new status.
For point 3 you will need developer's help. I can give you the starting point there is BADI: Amount to be collected. You will need to modify it to calculate the amount. As far as the display goes for that invoice item The process receivable list for the selected work list item is prepared in the two function modules - FDM_COLL_INVOICES_READ and FDM_COLL_INVOICES_READ_CLOSED. You will have to add implicit enhancement at the end of these two functions to display the amount as you want it.
Regards,
Prateek