I’m currently working on a new Kerning view.
Please see my reply here re: exporting fnt+png pairs.
If you edit a separate png and leave its fnt pair alone it should retain all the metadata when you reopen the font (select the fnt first and it should prompt you for the png).
You can toggle black/white color by pressing X (just like in Photoshop). When you start drawing on a filled pixel it will clear it and vice versa.
P switches the active tool to pen.
E switches the active tool to eraser.
If those shortcuts aren’t working please check your browser’s console for errors and report back.