[Galaxy S 5 Q&A, Help & Troubleshooting] Larnaen:
OK, this will sound crazy, but I actually got my "soft bricked" S5 to boot up again.

Here's what I did:

(*) Restored entire backup image from TWRP (every data type I had backed up).

(*) Unpacked the SamMobile device firmware (baseband id S902LUDUAOD3) and removed the following files: [aboot.mbn], [cache.img.ext4], [system.img.ext4], [tz.mbn]. Re-packed the remaining files into a tar file called like [base-firmware-slim.tar] containing just [boot.img], [modem.bin], [NON-HLOS.bin], [recovery.img], [rpm.mbn], [sbl1.mbn], and [sdi.mbn]. Pushed all those files via Odin v3.10.

(*) Restarted the phone. This time instead of just hitting a restart loop on the Samsung logo, it showed a Samsung screen, then played a boot sound, then showed the TracFone splash screen, then loaded the phone setup screen. I re-set-up the phone.

(*) Then it showed within the Android OS an angry-triangle notification like "UNAUTHORIZED ACTIONS HAVE BEEN DETECTED...". I tapped on this notification from the Android notification tray and the phone immediately rebooted itself into recovery mode, apparently performed a factory reset, then rebooted again.

My phone "about device" info now says

Baseband version: S902LUDUAOD3

Kernel version: 3.4.0-4647767

dpi@SWDD3915 #1

Sat Apr 11 00:01:45 KST 2015

Build number: KOT49H.S902LUDU3AOG2

SE for Android Status: Enforcing

SEPF_SM-S902L_4.4.2_0042

Sat Apr 11 00:01:16 2015

Hardware version: S901L.01

Device status: Official

So this device is in a weird state -- the bootloader and most of the System partition software come from the newer OG2 revision, but the kernel boot image is the older OD3 revision.

When I run the Samsung Kies 3 tool it tells me the device has "Current firmware version: S902LUDU3AOG2/S902LTFN3AOG2/S903LUDUAOD3/S903LUDU3AOG2 (TFN)", which is quite a mouthful but shows what's happened here.

I'm not sure if the "Odin mode" diagnostic info "RP SWREV: S1, T3, R1, A3, P1" is saying the same thing; hard to tell.

I tried reinstalling TWRP over my recovery partition, then using TWRP to try to restore the backed-up "modem" and "EFS" settings; this seemed to work ok and said it wrote over my modem after about 4 minutes, but even after I do this I still have the old baseband version and kernel version. Unclear why the restored TWRP backup didn't "work". I'm not sure whether anything in the TWRP backup of my pre-changes phone contained the LTE modem info (the non-HLOS.bin stuff); like, what TWRP backup file is supposed to supply stuff like tima_atn.mdt? Suspicious. After I did this stuff, the phone displayed "Device status: Custom", and it stayed that way even following a subsequent re-push of my [base-firmware-slim.tar]. So no OTA updates, but Straight Talk doesn't really do those...

It's not a brick, which is nice. The "unauthorized actions detected" dialog seems to be a permanent feature. I still probably return it given that it's in a very weird state.

(Edited: well, I returned the phone. That was an educational experiment. Too bad, at $229 the Straight Talk S5 was a great choice, but I really wanted a newer OS than 4.4 and couldn't find a way to make it work.)