However, I suspect there may be more overheads (and possible legal complications) associated with trying to run it as part of an existing charity
Given the current level of interest, the informal, small, and disconnected donor lotteries may be more efficient for the reasons you mentioned. My hunch is that donor lotteries could quickly grow to a non-trivial size, at which point I believe the economies of scale achieved by an institution would dominate.
you might think that the institutional reputation of GiveWell is more valuable than the individual reputations of people who work there
Given the current level of interest, the informal, small, and disconnected donor lotteries may be more efficient for the reasons you mentioned. My hunch is that donor lotteries could quickly grow to a non-trivial size, at which point I believe the economies of scale achieved by an institution would dominate.
Yes.