Only thing with mdf is Ive had problems tearing it up with mounting screws because it can be soft. And then when you start pulling on the armrest it rips the mdf apart even more. So Id make sure your screwing into plenty of meat on the armrest to avoid that. Or make them out of a different material other than mdf.
Door pull cups screwed directly into an L bracket might help take a lot of pressure off the armrests. Youve already cut out your pulls on the armrests though so that might not work. Just throwing out some ideas.