I, too, question whether you need any more seed at all. Sounds like you are already a couple mowings behind. Mowing at least twice a week for the rest of this year will make a big difference. If you have some low spots you want to fix, consider filling the, with your soil/peat/compost mix and topping those areas off with seed (that matches what you already used) that you have pre-germinated in damp soil mixture. This can be done in a wheelbarrow or several 5 gallon pails. Those spots may need to be mulched (grass clippings, straw, PennMulch, etc.) to assure that they grow and catch up to the rest of the lawn.
If you have been hesitant to mow because you feel it is too soft that is all the more reason not to run a slice seeder or aerator through it. If you are able to lightly roll it, it might make you feel better about running a mower across it. Chances are it is fine as is, in terms of firmness, with that much top growth.