The protective order obligates them to hold them until the civil complaint has been determined. In real terms, that means until it's been established - either by trial or by a plea - that any of the offences which allow confiscation of the assets (including the BTC) have been committed.
The spokesperson said that they would "probably" liquidate the seized BTC. They could, of course, fake the sale and hold onto all of them for use in future law enforcement operations.
If they are sold, it will likely be by public auction and not some method which allows purchasers to remain anonymous.
They might have to hold onto them until all the appeals are exhausted, otherwise they could be stuck in a situation of having to pay the cash equivalent (either for the price that they sold it at or the price that they are at when they lose the appeal, although the latter seems less likely).
How would faking the sale work? They'd say they sold them and hold them for operations? That strikes me as questionable compared to just buying new "untainted" BTC to use. This is especially true since BTC is very traceable by design so unless the FBI is willing to launder the money through a mixer, which would be difficult, time consuming, and expensive (since each mixer takes a cut) it would be useless for operations.
I have to agree with you n your last point. They'd go for a discount which would make someone a quick profit since they could, and if they were smart would, immediately turn around and sell them as the market will bear on MtGox.