You can try compressing png files; it will help with gwes some (I used a free app called PNG Gauntlet, or something like that, and it was pretty good). It's best to test out the PNG files before cooking and flashing (just copy them into \windows and soft reset), because maybe 1/100 will cause issues; the softkeybar gets messed up when it's compressed, as does the background in scroll bars and the pivot bar in menus (basically, the same as the soft key bar).
The easy thing is just to end your experiment with manila. It sucks, as you are discovering. Also, the 6.5 lockscreen uses a boat-load of RAM (in gwes), so you can disable it (saves 5-10 MB). Finally, you can save a couple MB of RAM in gwes by getting rid of all start icon/png images. Just use the native icons from the .exe's or cpl's; reshack any apps that don't have icons with ico files. You need to delete all of the Icon values in the startinfo registry, and you can delete all of the start_png files. One day, I decided to delete all of those Icon registry keys in the start registry, just to see what the start menu would look like, and I was surprised to find I had maybe 2 MB more ram after a reset. I also liked the looks of a lot of the control panel icons. They're old style, but I like the old style looks myself.