Not sure why your rates are so much higher with DD added.
You've gotten some good suggestions.
I agree buying tickets from UTis probably best option in your case, plus the split stay if you don't want to pay extra adult fee the whole time. (You could two bookings at ne hotel, or even two different hotels if you prefer.) We love split stays! PM me if you want suggestions on making them work. Disney will transfer bags for free.
Also, if you end up with two stays, both at same resort, let Disney know and they can usually make them a 'continuing ' reservation- so you get to keep the same room. (though I'm not sure they promise they will do it.)
Disney doesn't make it easy to figure out the advantage on package discounts, but I agree often room only is same/best as any other toffers.
One more option you may want to try is a WDW authorized
travel agent. (though maybe still buy tickets yourself).
AAA is one option, if you have it.