I'm not sure what to say about your dog not having interest in frozen birds. Does she get birdy at the scent of a bird? Showing her some live birds might be the best solution.
My golden was hesitant to work into thicker cover at first. I taught her that's where she's going to find them and over time she became less hesitant. When we do dummy work I throw them into thick cover. When I plant birds for her, I put them in thick cover. Over time she's come to learn that she won't find birds in lawn high grass - she'll find them in the tall, thick grass, the middle of large bushes, under the conifers, etc. It's further reenforced once it's actually hunting season and she's finding the wild birds in the cover. IMO you should practice like you it's the real thing.
*However, before you work on the thick cover, I'd first simply focus on retrieving frozen birds. Teaching a dog is allowing them to learn a series of building blocks. Don't expect too much of her too soon and keep it simple before you start complicating things with cover.
Also it's kind of late into the season to be working on some of this and expecting it to carry over immediately to the field - IME the best training comes in the winter/spring, then we're off in the summer with the bugs and heat, reenforcement in the fall, and then it's hunting season again.
FWIW, I've got a few birds in the freezer that are 2 seasons old now simply because I didn't get around to using them just yet. The ones I do use for training I just throw out once they get ragged. The scent doesn't seem to become lost if you keep them wrapped up, but I also sometimes add a little liquid bird scent to them depending on what we're doing.