The chargriller offset has a slide out drawer / ash pan. Does yours have this ?
Technically there isn't a real problem with air intake over the charcoal albeit is not as common. It's the same configuration as a fireplace. google Hearth Botom Grate
The air does get heated and creates smoke.
Placing the charcoal in the food chamber would make it draw air from the gaps in there
Try placing the charcoal as designed in the fire box and your control issues will be less, the food chamber will be exhausting smoke and not drawing in air. The gaps from the bolt issues can be corrected with foil as a gasket. Do some small cooks and learn what your pit likes to do. Note I'm betting it will not like 225 but that's not the magic number.... Find what it likes and let her run at that temp. Then adjust your cook times for what temps your rig can manage. It's not rocket science it's BBQ
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