Edge normals (hard and soft) should have absolutely no effect on smooth preview mode. When you switch to smooth preview mode (3-key) ALL the edge normals are softened and the model is subdivided 2 times (by default).

So there is no creasing as a result of hardened edge normals. The only reason edges would appear sharp would be if surfaces have reversed normals, there are co-resident edges, or other non-manifold geometry.

Do the following things...

First just simply harden all the edges (select the object and normals > harden) so in unsmoothed mode you can see all the faceted poly edges and all the open gl shading artifacts will go away. I hate modeling with softened edge normals because you get all kinds of open gl render artifacts and it's hard to visualize the actual poly surface.

Then do the following things...

1. Under lighting uncheck two-sided lighting. All surfaces with reversed normals will turn black.

2. Go to display > polygons > soft/hard edges. This will display softened edge normals as dashed lines and hardened edge normals with solid lines. Once again edge normals do not affect edges in smooth preview mode.

3. Go to mesh > cleanup and fix non-manifold geo, faces with zero surface area, and edges with zero length.

If you wish you can save the model out as a .ma change the .ma to .txt and upload it and I will be happy to look at it.

When merging poly surfaces Maya will always soften / harden edges in the combined parts using the default 30 degree angle to determine whether to soften or harden an edge normal. I like to keep the normals > harden, soften, and set angle tools on my shelf and harden all the normals as I am modeling so I can see all the poly edges clearly as I model.