Learn English – What does it mean to “eat glue”

You've pretty much got it with "patchwork or a makeshift measure." A very similar usage of the phrase can be found in a 2008 article from Advertising Age: "Team McCain Reclaims Lead With 'Spit and Glue'". Reading that article might give you a clearer idea of the full meaning of the idiom. You can think of "spit and glue" as "whatever's at hand, though it might not be the ideal or preferred solution."

In the passage you quoted, the full phrase is actually "held together with spit and glue"--it's just confusing because "the fiscal equivalent" intrudes into the middle of the phrase. Something that's held together with spit and glue (or, as Kit mentioned, spit and baling wire) has been patched together to work for the short-term, but it's not very strong or stable, and may not last for long.

A more common idiom is being knee deep or up to your knees in something, meaning that something reaches one's knees. When you are up to your knees in water, you have gone in to a point that you are definitely wet, and more importantly, there is so much water around, that walking gets more difficult (you move from walking to wading).

It is applied to other things as well, meaning that you are surrounded by a large amount of it, literally or figuratively (knee deep in sh!t means you are in a bunch of trouble).

In the same way, the person who said We'll be up to our butts in corpses means that once they are done with what they are going to do, corpses will be piled up around them up to the level of their behinds. Whether that will be literally the case is not relevant: the message is that there will be a fair amount of dead people, once they are done.

Note that the sentence as you quoted it is ungrammatical. It may be a typo on your side or an oversight of the editor, but it should either be we'll be up to our butts in corpses, or I'll be up to my but in corpses. It escapes me why one person would refer to multiple butts in this way.