Startup Hangs on Blue Screen, Won't Proceed to Desktop

G5 dual 2.0 GHz, 5Gb Ram, Running 10.4.11.
Before anyone asks, I need a Tiger volume to run classic programs. My B drive which I'm working from now, is Leopard 10.5.8.
My Tiger install disk is ruined, so I cannot do a reinstall of Tiger. I have to repair the problem.

My startup volume is one of four partitions on my main Seagate drive, but the only bootable partition.
After a multi-tab (10tabs) firefox session, I did a restart & it progressed to a plain blue screen & then stopped.
The cursor can move, but no desktop background pix, and I can't do anything except a hard shutdown.

I also did open and use by accident, an old old (probably 10.3 or earlier) free multi app called digitools, which is gone & has no support pages. I cannot find a plist for it, it must be under a name other than Digitools. Firefox & digitools were the only apps running before the restart that resulted in my problem.

Here's what I've done so far:
Ran fsck (reported startup volume was ok)
Rebooted from my B drive, then ran Techtool, Disk Warrior and Disk Utility (verify/repair disk). Techtool found no problems in volume OR files tests. DW found minor icon issues & replaced the directory. I do both these things monthly. Then I repaired permissions on the "problem" Volume. This was the result:
Repairing permissions for “My HD”
Determining correct file permissions.
User differs on ./Users/Shared/SC Info/SC Info.sidb, should be 0, owner is 501.
Owner and group corrected on ./Users/Shared/SC Info/SC Info.sidb.
Permissions corrected on ./Users/Shared/SC Info/SC Info.sidb.
The permissions have been verified or repaired on the selected volume.

I then rebooted from my firewire external drive (a sorta monthly backup of the problem volume). This was successful, everything ran nominally. I ran TT & DW on the problem volume, no problems found.
I restarted from the problem volume: same result: startup to blue screen and no further.

I can't boot into single user mode (same blue screen)
I can boot into safe mode ONLY to a "test admin account" I created last year to trouble shoot a completely separate issue.
When I logged out & tried to login in safe mode to my main account, same blue screen, cursor moves but nothing else.

Here's what I think I know. All my hardware is fine. Since I CAN boot into safe mode on the problem volume... but ONLY to the test user account, the problem has to be with my main user account (user caches, corrupted user prefs or user startup items, or other corrupted files etc).

I can't delete anything in my main user account from safe mode (obviously).
From my B Drive I can see open and otherwise interact with
ALL my files on the problem user account
(which is probably why all the file structure tests show ok). So I'm thinking it's a plist, a cache, a font, a startup item, something in the user account or the user library (maybe the system library, but if so, why would safe mode on the test account get me to the desktop but not safe mode with my normal account?).

I wanted to try Onyx to clear all the caches, recently it worked perfectly on a different problem, but you can only run Onyx from the volume you want to work on. I can't run it from my B drive to affect my problem volume.

From the above info what the best course to find the offending files that are preventing a reboot or startup from getting me to my desktop?
These hard shutdowns are murder on the system.

I do have a 2 month old backup of the problem volume. I don't want to recover from that backup because a ton of things have happened since the last backup (I know, I know), including taxes & music editing. That would be the very very last resort if I can't repair the problem. I think it can be repaired because it seems to me there's plenty to be tried before I have to resort to that option.

I can operate & follow (copy/paste) terminal commands, so feel free to suggest, but I don't know the terminal well, and the commands obviously would have to affect the problem volume, not my current B drive startup volume.
Many thanks in advance.