Basically, it all works now except for doing the actual AJAX request (10 minute job) and adding blockinstances to an empty column. The hotzones at the bottom of columns have been extended to the end of the content, for ease of use.

Without the outlines, and with a 'snap' of five, it doesn't perform too badly, but there is probably lots of improvement to be had still. Some kind of data structure storing the layout of columns might be useful, but also may cause problems if it gets out of sync.