I've been looking through the CBA on this and it appears that roster charges only apply if a team is under 12 players. Apron calculations don't take into account roster charges above 12, a team is just SOL if they are bumping up against the apron with only 12 players. But that doesn't really help explain the numbers Spotrac is giving us.
Edit: OK, so I figured out where the Spotrac numbers are coming from. Domas ($1.3M), DDR ($1M), and Lyles ($1.2M) have some "unlikely bonuses" written into their contracts. These unlikely bonuses have to be placed in a team's apron salary because they could potentially be paid out. That's why our apron hit is $3.5M more than our salary cap number.
So as it stands we have $4.9M in apron space. If we were to do the Huerter/Cam trade I've talked about, we would have to add in Colby to stay under the apron, we would have 12 players under contract, and we would be unable to sign any free agents for the #13-15 roster spots, even at minimum salary. That seems pretty unlikely when it's put that way. So it probably is more likely that we cleared the McDaniels space to add two guys on ~min deals to fill our currently vacant #14 and #15 slots.