First of all...lets look at the cone. It should have at least one inch of clearance around the neck...not so loose that she can shake it off, but not so tight that it rubs. Now as for the edge that is nearest her skin (and the outer edge as well) they need to be wrapped with several layers of duct tape, enough so that it isn't a raw blunt edge but one that is "cushioned" with at least three layers of cloth tape, and the duct tape is wide enough to hold on. The cone itself needs to be removed and cleaned a couple times a day, with either soap/water or Fantastic or something antibacterial...but not while she's wearing it. If its wet after she drinks, you'll need to reach inside the whole cone and wipe it down with absorbant paper towels. I know its trapping some moist air in or near her ears but you will have to wipe them out several times a day also, trying to keep them "dry" with cotton balls or gently using a gauze pad over your finger...gently because the gauze can be abrasive but it also cleans and absorbs.
Yes, by all means, don't hesitate to give her Benadryl to get her to quiet down. Its safer than Acepromazine. The Deramaxx is meant for pain and inflammation but adding 1-2 tabs of Benadryl shouldn't negate any of the effects of Deramaxx. The downside of Benadryl is that it can, and I say "can" cause some reduction in tear production, but you are supplementing the eyes with medication anyway now, right?!!
Remember that most oral a/b's take 48-72hrs to fully get going in fighting infection and you may be seeing the last of the resistant period...and she may soon be benefitting from a reduction in the ear infection and its resulting discharge.