I've apparently found the solution.
I hit on this solution when I discovered that at certain times of the day I could process fairly large files with Imagick on the live server, but then could not just a few minutes later. This suggested to me that memory was being consumed by others on the shared server at fully expected erratic and unpredictable intervals, and therefore if there were a way to allocate memory to the Imagick object before initializing a file in it, then it might work.
The trick is to explicitly allocate memory to the Imagick object after creating it using the setResourceLimit() function of the object, calling the 5 "RESOURCETYPE" values as shown below. What follows that is simply the resizing routine I perform on the image file, which was never the source of the problem.
[php]$im = new Imagick();
$im->resizeImage(528, null, Imagick::FILTER_LANCZOS2SHARP, 1);
I hope this is helpful to someone else, because I was banging my head against the wall trying to find a solution; Imagick/ImageMagic is indispensable to my site.