It will eventually send runners and fill in, but IME unless you are very dilligent with pruning your HC and keeping it short this will not be the last time you have the issue. My HC looked great, but everyday I would come home to find a corner of my carpet somewhere in the tank lifting up and floating. You need to keep cutting it short or the buoyancy of the leaves lift and uproot it.
You can do both very sloped and bottom at the same time, you just have to do it correctly. To really get this right you need a ton of time and patience.
You'll have to constantly wet the top part, but meanwhile maintain the lowest part of the tank to keep it above water and still damp. The best way to do that is probably to take a mister and spray over it generously in the morning/ evening or both (mostly on the top portion, where the steep slope is). Keeping the substrate very moist but not enough to flood the tank. Also keep seran wrap or plastic over the tank to keep humidity in.
That would take a while to fill in. It really depends on how you feel about it. I would drain and replant. If you go this route, only fill the water line to the front and just mist the tank a few time a day. I used a turkey baser to suck up the water that started to puddle in the front. This would help your HC root and spread faster. Good luck and BTW, the hard scape looks great. It's going to look awesome once it's all filled in.