[Verizon Galaxy S 5 Q&A, Help & Troubleshooting] 1CDT:
So I will keep working on this, but I can’t currently verify that the bootloader doesn’t need to be updated on SM-G900V, based on the work I did today. So far I can only verify that LineageOS will boot and work with an NCG bootloader, except for some bug where it destroys data in the EFS partition which I suppose is not actually bootloader-related :)

First, after backing everything up in TWRP, I started with the baseband modem and firmware updates to QL1. The steps for this were:

1. Boot into download mode (vol dn + home + power)

2. Run heimdall flash --RECOVERY recovery.img --BOOT boot.img --no-reboot using the files from the stock QL1 image (any of them will do)

3. Hold power button to turn off phone

4. Pull battery

5. Boot into download mode

6. Verify that “Current Binary” is ”Samsung Official”

7. Run heimdall flash --MODEM modem.bin --APNHLOS NON-HLOS.bin --RPM rpm.mbn --SBL1 sbl1.mbn --DBI sdi.mbn --TZ tz.mbn using the files from the stock QL1 image (or from the baseband firmware thread, they are the same)

8. Hold power button to turn off phone

9. Pull battery

10. Boot phone back into download mode

11. Run heimdall flash --RECOVERY twrp.img to reinstall TWRP

12. Hold power button to turn off phone

13. Boot into recovery (vol up + home + power) to verify the flash and to ensure it doesn’t get erased

14. Reboot to system

This caused every application to crash on boot in the already-installed NK2 system ROM. I don’t know if I did something wrong, or if they are just incompatible; I did wipe cache and dalvik cache from TWRP, but that didn’t make things work. (I had the same problem when I had to roll back everything later; more on that in a bit.)

Since everything suddenly was broken, I assumed that the baseband update must have been successful (I later verified in LineageOS that it was indeed successful), so I followed the LineageOS instructions to sideload LineageOS and Open GApps from TWRP. This was successful and the OS installed and booted to the setup wizard.

The first problem I encountered at this point was that the mobile network wasn’t connecting during the setup wizard. I skipped this step of the wizard and continued on to configuring the OS settings. Eventually the mobile network connected while I was doing that.

When I started installing apps, I noticed that it took a very long time to receive SMS from the network. Upon investigation I discovered that LineageOS was using legacy CDMA for voice and SMS. I did research and discovered that LineageOS does not, and apparently never will, support VoLTE on klte. Since this is a non-starter for me (Verizon will be LTE-only by the end of next year, so I have no idea how LineageOS klte will exist at that point) I opted to wipe and load stock QL1.

While preparing to load stock QL1, I restarted the phone accidentally, and noticed that I’d permanently lost mobile network connectivity. I tried restoring my EFS backup from TWRP; this didn’t seem to fix the problem. I was going to install stock QL1 system anyway due to the VoLTE problem so I didn’t think about it any more.

At this point I followed these steps to flash stock QL1:

1. Boot to download mode

2. Run heimdall --BOOT boot.img --SYSTEM system.img --no-reboot using files from the stock QL1 image

3. Turn off phone

4. Reboot to recovery

5. Wipe data, cache, dalvik cache

6. Reboot to system

After 15 minutes at the Verizon logo while the dalvik cache was built, the setup wizard started and mobile network connection was working and I was able to complete initial setup for stock QL1. However, the system was not OK:

1. Wifi would not enable

2. The back and menu buttons did not work

3. When the phone locked, after a while, the notification LED turned red and the phone wouldn’t respond to any button presses (I had to pull the battery; this happened multiple times)

At this point I needed a working phone, so I found an NK2 stock image (this was difficult because all the links on xda-developers are dead and sammobile wants money for these old versions, so someone might want to reupload these!) and ran these steps to roll back:

1. The same steps above for installing the baseband modem and firmware, except using NK2 images and firmware

2. Boot to recovery

3. Wipe data, cache, dalvik cache

4. Restore NK2 TWRP backups of System, Boot, Data, EFS

5. Reboot to system

Instead of being fully restored, every app was crashing on boot again, like when I had updated the QL1 firmware and rebooted into the old NK2 system. I was finally able to get my full backup restored successfully by following these extra steps:

1. Boot to TWRP

2. Wipe Data

3. Reboot to system, until the setup wizard starts

4. Turn off the phone without running the wizard

5. Boot to TWRP

6. Restore Data

7. Reboot to system, everything is OK now

The hard buttons and wifi problem are noted by [MENTION=1949695]jrkruse[/MENTION] on the unlocking the bootloader thread. I had to get my phone back in a working state for tomorrow so I didn’t try the instructions to reflash the PD1 boot+recovery+firmware. I’m not actually sure if it’s correct today to reflash PD1 firmware instead of QL1 firmware; clarity here would be helpful. If anyone also sees a clear mistake in the steps I outlined above, I would be grateful to know that.

If I can’t get stock QL1 to not be broken, and it’s due to the bootloader requirement, then I guess I am stuck unless I start messing with the bootloader (given the “We still are unsure if changing the CID causes app store, verification, activation, provision, or other issues, everything you do is at your own risk!” warning, this means me trying to transplant my dev signature onto the EMMC 15 bootloaders even though my signature is a different size). I’m pretty terrified of doing that since there’s conflicting information about whether it’s possible to flash an old bootloader once you’ve upgraded past certain versions, and I haven’t learned yet how bricked the phone becomes if a bad bootloader is flashed. I know it’s not possible to flash old bootloaders on a retail device; is that true on Dev Edition devices too?

Thanks again for your help! I wish I had more positive news.