1. What percentage of liquidity are we still getting from Stamp on average?
I do not see Bitfinex integrated with Bitstamp anymore. I think all the liquidity is at Bitfinex only.
2. How much of Finex's holdings are kept in the Stamp account at any time on average?
With the platforms no longer integrated, there is no reason to hold BTC there.
For the remaining concerns, the simplest solution is to simply hire someone like Andreas Antonopoulos to come validate your BTC balances, fiat balances (ie witness you login to your bank), Stamp balances
That is a typical financial audit, which any accounting firm can do. The key is to get a world wide recognized firm, not a local CPA with no name recognition. Too much potential for abuse (buying the opinion) with a no-name firm. This also needs to be done on a regular basis (at least quarterly, preferably monthly).
, your facility security, infrastructure locations/connectivity, cold storage, etc... like he did for Coinbase last week. A regular auditor wouldn't have the experience to validate many of these effectively.
That is a different type audit, and yes you need a specialist for this. This only has to be done once every year or so.