If I understand your question, you're asking which detail you should use at your mezzanine piers; column block-outs as shown top or rebar as shown below.
Either of the two column block-out details will definitely work. These are formed pockets that are filled in with a second concrete placement. Obviously a lot of work and not great aesthetically.
The lower detail showing rebar at a re-entrant corner is a 50/50 proposition at best. It's a very common detail in commercial construction and isn't very effective at preventing cracks. The engineer is basically admitting the slab is going to crack and sticking some rebar in to keep them tight.
As a third option, you could wrap the piers with expansion material to 'soften' the corners and provide for some movement. This might be enough to prevent cracks
You could also cut your control joints perpendicular to the post corners in a pinwheel fashion. This would be a simpler, better looking detail that would give you a reasonable chance of avoiding a random crack.
As a famous man once said, "Re-entrant corners ****". Oh wait....that was me. Well I was right, they do. It is very difficult to prevent cracks when this condition occurs and occasionally they'll show up despite your best efforts. Usually right beside a control joint that was supposed to prevent it. At the end of the day, the best way to control cracks is too avoid them so try to place in moderate temps if possible and pay close attention to slump, curing, etc.
Good luck and congrats!
o 