A 3 tiered system for this is probably the best idea. I'm always using this approach with MC to emulate the relationship between [Album], [Album Artist], and [Album Artist (auto)]. [Album Artist (auto)] takes the value that's in [Album Artist]. If [Album Artist] is empty, then it takes the value in [Album] instead.
This is really elegant because you can just use [Album Artist (auto)] anywhere you need it and it will normally use the value of [Album]. But if you have exceptions (like albums with guest artists), then you can put your exception in [Album Artist] and it will just magically show up in [Album Artist (auto)]. With that logic in mind, I propose:
[Date Acquired]
[Date Acquired Override]
[Date Imported]
[Date Acquired] would be your master sort field for sorting albums by the date that you got them. It would take the value of [Date Acquired Override] if it exists. If not, then it takes the value of [Date Imported]. That way it always has the same value as [Date Imported], but you can OVERRIDE it with the [Date Acquired Override] field any time you need to.
Brian.