The fact that the case is still listed as open may be an issue, so good call on getting an actual attorney involved.
Best rule of thumb I've picked up on this board is to assume that they do have access to your criminal records and to err on the side of disclosing more than less.

What greglvnv wrote is also refers to something I've learned here - just because a record has been expunged from your state/local systems does NOT necessarily mean it's been expunged from related federal databases. So it may be gone from your state system, but still be in NCIC or whatever other databases CBP may include in your background check.
I would dispute his first comment that your SO will get denied with that on her record. There are plenty of cases here where someone has disclosed a previous conviction and still been approved for GE. I don't know what factors they consider, but I would guess that it may be based on how many times, how long ago, type of offense, age at the time of arrest/conviction, etc.