I’m not convinced that showing the multi columns within the confines of the current page layout are necessary. maybe others have different experiences, but I pay bills out of one account. But,on the other hand, these are categories we are talking about here, not always specific bills so maybe I can see paying His phone bill out of His checking account and Her phone bill out of Her account. Both fall under the Phone category.
What about just changing the way you can view that information. Instead of changing the entire page layout to enable the columns(which is a huge waste of space if you have to allocate area to empty or non-participating accounts), why not have a hover popup when you mouse over each category that shows quick summary info on that category. The popup should appear just under the category so the checkbox activation is still enabled on clicking. As you moved down the page, once you leave the actual category area (not the popup) that popup disappears. If you moved over another category, popup that categories quick summary.
This would preserve your current layout and functions and simply add a popup feature (which could be used for lots of other things as well).
If you really wanted to provide an account breakdown, do it like the charts where you pop up a window to show the breakdowns for the month. You could also include a link in that window to download it as a tab-delimited text file or just straight text.
The quick summary popup could include all accounts that contributed to that category for the month, the current amount, the budgeted amount, maybe even a tight/small review of the past couple months for this category. Size/space used isn’t as important since its going away anyways as soon as the mouse leaves the category label area.