Firstly, if you already have it installed of course it wouldn't install it again without either you purging it first or using the --reinstall flag.

Secondly, mcetools is to control mce which can only be done if it is installed from a trusted source. By being installed from unknown origin it is unable to do so. Inception or open mode's fixed origin gives it a trusted origin.

An alternative I think is to change the origin of the existing installation of mcetools in refhashlist to a trusted origin and re-signing refhashlist. This method is pretty easy to give you a MALF if you don't know what you are doing.