Quick to make ends meet...
You can delete half your model, turn on your floor on the correct axis mask out all verts that are on the edge,
invert your mask and use the move tool to move them and make sure they cross the edge. once that is done you have 2 options, using the clip brush you can snap them.
But a better option is to use the mirror and weld feature.
In regards to subdividing, Its easier to duplicate your original mesh. Now decimate and do your qremesher on the surface no need to subdivide.
After your qremesher start subdividing and applying projections.