Thank you for buying the font pack!
You can add latin accents to any of the fonts that don't have them yes - to be honest you can edit the .ttf or .otf file yourself if you want as long as you don't redistribute it afterwards.
You can definitely edit any version converted to bitmap too, since you might want to make logos with different styles and stuff, so editing the outline of the font in sprites and graphics is totally fine.
You don't need to credit us, but if you want to you can just write "Font by Chequered Ink" or something!
Finally, your English is fantastic no need to apologise!
All the best,
Daniel