Importing Subcontracts into Acumatica

1. Do you use the Acumatica standard Subcontract import scenarios for importing subcontract balances? - Yes, only open subcontractor orders should be imported into Acumatica and the retainage if applicable should be set on the project itself. I have a template and a working import scenario. 
2. When importing subcontract balances, how do you update subcontract Billed Quantity and Amount? Is it through a customization? - Only open amounts should be imported. The only way to update it is to import AP invoices and associate it with Subcontractor number and line number. However, those AP invoices cannot be imported in migration mode if they have retainage amounts. 
3. Do you import AP bills with unreleased retainage, change orders, compliance records linked to the subcontract? Only for completed information, but for open ones do not associate it with subcontractor number. Any AP bills with retainage must be imported with migration mode off and the debit and credit GL accounts must be same assuming GL is imported from trial balances
4. What are the difficulties you currently face in importing subcontracts information into Acumatica? None

Your responses will greatly help our engineering team in building the right solution.