Please excuse my ignorance on the matter of updates and the way the US cellular network providers work, but why don't you buy your phones unlocked? Would that solve the 'carrier testing' issue? Would you then be able to get updates directly from Microsoft?
Here in the UK I can walk into any Carphone Warehouse outlet and no matter what phone I buy, whether on PAYG or on Contract, it will be unlocked - identical to, but cheaper than, a 'sim free' handset.