Sorry, there is no easy answer to either of them. Not something i have tested or needed.

1) I was hoping LazOpenGLContext would have had fullscreen property, but it doesn't. Lazarus crew propably relied on SDL with it, of which i have no experience. It is possible to adapt SDL window and use nxPascal with it in NX_CUSTOM_WINDOW mode, explained in demos and begin of nxGL.pas, but this specific window i haven't tested.

So what remains is non-crossplatform ways, like directly using WinAPI. Maximizing window, showing it on top of everything and hiding taskbar. As you can imagine that doesn't cope well with Linux with multiple taskbars and other widgetsets.
There was some discussion for windows. You can find more with search terms something like "delphi fullscreen graphics". Maybe if it's something simple, i might integrate it to nxPascal itself.

2) At this point there is no font saving or loading. There is another inherited constructor to TGLFont class, which takes just texture-size as parameter (must be square), but loading just that you can't tell how wide each character is. That needs to be loaded from file, or calculated on load from TCanvas fonts. Relevant data goes to charW: array[32..255] of shortint; and textureI: integer; which is index to manually loaded font texture (which is only needed if you want to call font's SetTexture-method). Basically the font class itself supports any kind of fonts, even if they don't exist in Windows system at all. To use such fonts, an editor should propably created, or possibility to use some existing font format of other editor, that includes texture file.