I had a situation once, not the same, but we resolved issues by using exmerge to extract healthy data from one IS and then recreated and re-imported the data to a second IS.
Exmerge breaks the single instance, so the IS will typically increase in size by anything from 60% to 200%, depending on the exact content.
A similar option here, depending on the version of Exchange would be to create a second storage group with a new mailbox store, and use the move mailbox wizard. This in theory will only move the mailbox data that you are seeing in ESM, and whatever data that we are not seeing will be left behind. Move mailbox will retain single instance. Just checked your first post, SBS runs Standard edition, so this ins't a usefull suggestion sorry.
We're missing something obvious. Did the last online backup run. The online backup backs up the database 4k (1 page) at a time, the backup job checks the physical page number, the logical page number and performs a data checksum, it compares the calculated checksum with the checksum in the page header, and verifies that the logical page number is the physical page number -2, (the first 8k or 2 pages are the database headers) Then it backs up that page, and moves on to the next.
Bottom line, if the online backup has run, at a page level the database is healthy, there is no page level corruption.
At the next level up, Eseutil or Isinteg should fix any other issues, Eseutil is a little more severe, it simply eats corruption, it's more of a surgical excision of bad data, and you can have a situation where the page that Eseutil removes is the top of a folder structure, in which case you may have lost an entire folder or even a mailbox.
I always tell people that 90% of the time Eseutil or Isinteg will fix a problem, but 10% of the time they can make it 90% worse.
But we've got a situation here where Exchange thinks it fine, so chances are we're missing something.
Over the last number of years I've learned to give Exchange a grudging respect, it's usually doing what you've asked it, in many cases you just have asked it to do something odd...
Here's one out of left field...