1) Airstone will not impact worms
2) I am not sure if assassins will tackle detritus worms, maybe someone else has experience with this.
3) All depends on your betta's disposition as to whether additions will work.
4) This is the route I would take. Dose the tank with Fenbendazole. Your fish will be fine, snails will need to be removed though. Very little time and effort required with this method. Dose the tank, wait a few days, do a water change. With dirt, removing the dead worms from the substrate will be an issue and you will have decay which can lead to some issues potentially if there's a ton of them...that or it'll just feed the dirt.
5) I use eco complete, I just take my power-head in my hand and point it into areas where detritus may have built up, get it into the water column before a large water change. If your cap is 1"+, you should be fine with this method.