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The best way to create the single boundary file for the polygons would be to use the Summarize Tool. Select the Spatial Object field, then click on Add-->Spatial-->Create Convex Hull. The ultimate result will be a single polygon representing the outtermost boundaries of the subject polygons.
Yes - positive and then negative buffer is almost certainly the answer to concave hull. By changing the size (in miles) of the buffers, it will determine how much of the gaps it will fill in. If you want the outline to track exactly around the outer portions, the positive and negative buffers should be the same size. If you want it to extend outside a little (like the red line above), the positive buffer should be bigger than the negative.