Sub Pabel Install, NEC 250.24 question, Mixed ground/neutral at main
OK the NEC and AHJ have always allowed the grounding conductors (Ground) and the grounded conductor (Neutral) to be connected to the neutral bus bar in the main panelboard or loadcenter. You are correct the busses are interlocked and bonded. The only reason some electricians put the ground on one side and the neutral on the other is because of the "workmanlike manner" clause. I have never seen where mixing them has ever caused any problems.

Now, your sub panel is not covered under this rule due to the single point of ground rule. You need to install a grounding bus and an isolated neutral bus. Each bus must be connected from the service panel with a corresponding conductor and you do not bond the sub panel neutral. This is true with any other device, equipment or sub panel installed downstream of the service panel.