The soil is heavy black clay and I have been using the organic program for approx 10 yrs. When I walk thru the yard with white socks, especially where the dogs park themselves during the day, I get nothing on the socks. This seems to indicate no heavy flea concentraton. With three 50# plus dogs, you can imagine the compactness of the soil. I hadn't thought of adding the molasses, though.
If you can do it, I think it would be a good idea to aerate. (If you've been organic that long, maybe you have been aerating.) The current belief on beneficial nematodes seems to be that they migrate less/are less active in compact soil. If aeration is necessary before you mulch and if mechanical aeration is impractical, you could try the bio soil treatments (like Medina?) in conjunction with the molasses. If the soil conditions are okay and not too dry, you really shouldn't need another nematode application; if the soil conditions are detrimental, another nematode application probably won't do much good. If you can, keep the soil moist enough to support plant growth, address the compaction, and monitor the flea population for new hatches (a flea comb can be a good tool for monitoring the population on the dogs). Of course, if the dogs come inside, then you have the indoor flea cycle to deal with. Finally, if the dogs had fleas, watch for tapeworms as a not unusual side effect. It probably wouldn't hurt to feed some food grade diatomaceous earth, if you aren't already or if it isn't in the food.