Is the claybreaker going to "condition" the clay to make it viable for fescue or do I still need to topdress it with topsoil?

Yes, Claybreaker is a soil conditioner, it helps the clay become productive to grow a great lawn. Unfortunately, it's not one application and you are done, it takes many years to build up the organic matter to compliment clay. It can start by not bagging your clippings, mow your leaves rather than bagging them, adding more Claybreaker. Clay actually has some benefits, it's great for chemical conductivity, a little plant food goes a long way, because the particles hold-onto nutrients. In as far as Top Soil, it's for maintenance, leveling the soil, filing in low spots. If you want to Topdress, use specific products such as Lawn Soil which is designed for Topdressing, and another good product for Topdressing is peat moss. In starting a lawn, I see no benefit from Top Soil. And lastly, if you are sowing the lawn this spring, seed germination is usually lower in spring than fall due to natural weather conditions, read the seed label as to what rate you should apply in spring to get the germination percentage for a full lawn. Good Luck,