I would try splitting the model into sub-components rather than attempting one offset (e.g. rim, floor, arch, balcony, doorway) to reduce the complexity of what you are working with in cleanup.
Looking at the model, there are elements that look like they should be simple forms but are slightly off. For example the rim around the front looks like a circular arc but its bounding box isn’t quite square, the outer faces of the kick-up extensions towards the rear of the side rims look planar but are not. I find that solids or faces created by offsetting the model’s individual surfaces fail to union or split. If these small deviations are unintended then using simpler forms may fix that. And you can then construct some of you rear surfaces from curves rather than offsets (the offset command doesn’t seem to recognise when it is, say, replicating a circular arc and uses more construction points than another arc would need, so you are adding complexity).