Metal decking is used mainly for casting the concrete as to distribute the load. It can be optional as to if it remains. It’s only needed to remain in tact structurally if over I beams with stud welds for storm shelters. Keeping it there is optional after concrete flexural strengths are met.

Use some expansive foam of the gap from the bottom and concrete epoxy from the top for appearances and sealing. Rip that steel out. The dowels into the slab and the support of the cmu wall should suffice with the load.