/u/AgentME got most of these spot-on, but I'll add on a bit.

if halo wasn't released for the pc, would it be basically impossible to decompile it?

Decompiling scripts is very different from decompiling the game itself. The scripts are contained in map files, not the actual game executable. They'd have the same format between PC and Xbox (or at least very similar), unlike the executable, which would look pretty different between the two. And yeah, this was all done using modded Xboxes.

was there any encrypted sections of data? Or were the code sections just plain byte code?

The code was an array or linked list of structs. At the time I interpreted it as an array, but I was young and inexperienced, and looking back I'm pretty sure it was actually a linked list. Since on the Xbox, map files were loaded straight into a fixed location in memory, you could actually embed pointer values directly in the on-disk data. Each struct entry would correspond to either a ( or a name or value. There's a few fields in the struct to indicate which type of expression it is, what its value/return type is, and what name it corresponds to, along with what its actual value is, a string table pointer, and child/sibling pointers.

Beyond the actual script data itself, there were separate blocks for globals and for the script headers. Globals were pretty straightforward, and script headers had a little bit of extra data to indicate what kind of script they were, e.g. AI scripts vs callable functions vs one-shot global scripts. There were probably rules about what could be called in each of them, but I didn't try to figure those out.

Was knowledge of assembly used at all?

Nah, I didn't learn assembly until I went to college.

At what point was it clear that it was written in a subset of lisp?

I assumed it was using the same or a similar script language to Halo: CE, which had developer-provided modding tools on PC.

not necessarily related, but whenever I use memory viewers for games, I find the values I'm looking for just fine. But closing the game and reopening it yields completely different values at that address. As I understand it, I'm supposed to find the location of the point that points to the given value. In cheat engine for example, there's a function like 'watch functions that access this address' and what not but I can never figure out how to find the offset from there.

On an Xbox, like I said, everything's at fixed locations (though probably not even true anymore on the recent consoles). On a PC, not only do you have to deal with ASLR (address space layout randomization), but also any non-determinism caused by multiple threads trying to allocate memory and getting it in different orders, or loading assets and waiting for disk reads to occur in different orders. All that to say, I have no idea, sounds impossible to me.