I dug around a little more and found this page:
http://us.codejunkies.com/nds_actionreplay/update_info_us.asp
I has the link for the 1.60 bin above, as well as 1.54 and even a 2.00 link that gives a 404.
I just wish I had read things a liiiiittle more carefully yesterday. (you'd think the big red text would have been enough.) I'm stuck on 1.71 now and unable to input any more codes. I've contacted Datel about this and have not heard back yet.
I'm not expecting any response from them other than "lol, buy a new ARDSi" though.
Just to clarify, I have the v2 pass-through cart. I was on 1.54 and did the FIRM edit on the 1.71 bin. If only I had waited until I found the 1.60 I might not be in this situation.
Cephyros, do you have any insight on how difficult it would be to write a custom app to brute-force downgrade the firmware on the v2 cart? I've tried to look into this, but my skills are not honed to this particular kind of thing.