Widows who remarried were not eligible for pension support, so that would be why.
Since prisoner-of-war exchanges took place at Vicksburg MS on Sept. 24, 1862, it's impossible for this soldier to have gotten to Tennessee before the month ended. Soldiers died during the trip down the Mississippi, but the service file should conatin that information.
There were two Tennessee regiments numbered 48th, so we'd have to see the actual record to know which one was meant. Here's a history of Voorhies' Regiment --
There's also a relatively new book about Camp Douglas titled, "To Die in Chicago" by George Levy.
You asked about suggestions. Here's one -- next time you order a service file or pension application, please consider clicking on the link at the top of this page! You should get your copies in a much more timely manner, and orders placed here help support the message boards.