1) Unknown to me, but maybe the update of the component made him provide an option for those that would not need to install, since they are not going to use the features it is used for.
2) For me, it did not remove the already installed version, when the check-box was not checked. (I assume that it is not supposed to remove it, anyway.)
3) You could re-run the installer (or the latest version of it) and check the option. You can download the component. ImDisk3.0_Full.zip (or a later) contains a imdisk.inf which you have to right-click and then select install. (The removal works over the Windows Control panel, according to the readme)