>>One might think the above is pretty onerous but the alternative i.e. no confirmation, is unthinkable. Without confirmation the feds could manufacture anything they want and claim that's what they got from the phone, even if they couldn't decrypt it at all!
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>The contents of the phone could be corroborated by other evidence. It could be used to tell the security services where to look for "facts" but not be itself regarded as a fact.

Isn't that what they usually do with otherwise inadmissible evidence? Use it as a pretext to get a search warrant to confirm or abandon that line of investigation. An inadmissible wiretap may not be used as evidence, but if it describes the place with the loot and they find the loot at exactly the place, then that stash becomes regular evidence.