ckeditor allow all tags and their css classes etc and stop editor creating an empty paragraph along with option to insert paragraph when it is disabled

help.js

CKEDITOR.replace( 'ck-editor', {
  allowedContent: true
};

CKEDITOR.editorConfig = function( config ) {
config.enterMode = 2;
};

And this will give you both the <p> and <br> tag when needed but does not prevent
the start and stop <p> tag

CKEDITOR.editorConfig = function( config ) {
config.enterMode = CKEDITOR.ENTER_BR // pressing the ENTER Key puts the <br/> tag
config.shiftEnterMode = CKEDITOR.ENTER_P; //pressing the SHIFT + ENTER Keys puts the <p> tag
};