Change canvas size ERROR - always crops image, never adds borders

"I'm currently working with a large number of images where I repeatadly have to resize whatever resolution of each image to 1000x1000 pixles (filling in with white borders to maintain the proportions of the image when it's been resized). "

You are the the Apple Orange thing in the above statement.   Photoshop is a Pixels Editor  Canvas size is the number of pixels you have to store image details. These pixels is the bulk of the data in an imaged files. It what make files sized large.  To reduce the size of  files data compression is used  on the file's data for there is lots of redundant data in Pixels data that can be recorder in a compressed format and expanded back when read. 

Resolution is are Printer setting.  Resolution is the size Photoshop will Print an Image's  Pixels. So if  you have age Image camera 1000px by 1000px and the Print resolution is set to 300ppi.  Pixels are 1/300"x 1/300".  The Image prints  1000/300" by 1000/300"    3.333" by 3.333"  it the print resolution is 100ppi thes pixels are  1/100" x 1/100" your picture prints 10" x 10"   

So if the you change the number of pixels your image has to 1000px by 1000px if you are reduncin the number of pixels the imags has you are croping image content.  If you increase the number of Pixels you are adding empty canvas boarders. You can actually do both crop and add boarders all images do not have a 1:1 aspedt ratio.

To make any image 1000px by 1000px you need  resize the image  to fit into a 1000x1000 pixels canvas and then add amy missing canvas to make image be 1000x1000 pixels 1:1 square image

"Now, here's the problem, the resize canvas option is not working correctly (this is also what the adobe customer support agent established - didn't get the chance to resolve it though). All it can do is to crop into the image using the center as a reference point, ie leaving a tightly cropped original image, without resizing the content at all. "

Perhaps you misunderstand Canvas Size option? Canvas size will never interpolate or change pixels, it will only alter canvas or visible area, not image pixels. There are other options to change image dimensions like Fit Image which may offer solution for your problem. You can use Canvas Size afterwards to expand canvas to sides or exact dimensions. If you have mixed orientations of images to batch process then you may need to record multiple actions and to use Conditional actions.