Oh my well you mentioned your first problem which is that it didnt have any sunshine and the fertilizer may have made them stems mushy but someone else could probably be more help in the department but my dahlias are being finicky this year and not doing well at all maybe its something in the air...

As you may know, dahlias love sun, they do not like wet feet, so fast draining soil is best. Also, if you want to grow the same variety that you bought...seed is not the way. You can propagate from root division, root cuttings and the like. Basically it is like cloning. If you grow from seed, there is no guarantee what type of flower you will get! I would place the plant in a warm sunny location. Give it a mild fetilizing of an organic liquid like Dr. Earth. Trim off the dead parts. Dahlia are pretty resilient..Good Luck...