How many btc have been transferred in orphan blocks, but then never
included in any later block?
This is not a statistic that's normally published by anybody, as far as I can tell.
How many btc have been transferred in zero-confirmation transactions that have never been included in the blockchain? How could one find this info?
The data necessary to do this analysis is out there, though it's really hard to know if it's complete.
Blockchain.info has a list of orphaned blocks back to March of 2014. It's hard to prove whether this list is complete since orphaned blocks don't propagate well (hence their being orphaned). But given these 400 or so orphaned blocks, you can grab all the transaction hashes, see how many of these transaction hashes can not be found in other blocks and that should set an upper bound for possible fraud activity.
You'll also have to see if the transactions that spend certain inputs in the orphaned blocks end up going to the same outputs but have a different signature and thus a different transaction hash. You should eliminate those as well.
Finally, you'll have a set of possible fraudulent activity for the past 11 months or so. Not perfect, but given that very few have any incentive to keep orphaned blocks around, this is a good a method as any.