I'll try to make some sense of all this, but no promises
Regarding the WDIDLE3 version 126.96.36.199, it is clearly unsupported and unofficial. It might had been originated from WDC's support, but there is no way to download an official public version with relevant documentation.
Many tools are available with some "use at your own risk" note, but at least we all know they are "official". Although the origin of WDIDLE3 version 188.8.131.52 "sounds" legit by users' accounts, someone could very well argue that this is how "malware" is distributed too.
Some UBCD user might think that this new version has exactly the same purpose as the prior (official) one, but then perhaps there are differences or negative effects?. More importantly, we don't really know which HDD models are covered by this new version. My personal point of view in these cases is usually "first, do no harm".
Therefore, I tend to think that this WDIDLE3 version 184.108.40.206 should not be officially incorporated into UBCD, and instead should be left as a DIY customization.
Regarding the tests, WDC recommends that, when in doubt, the drive should be connected to an individual (S)ATA controller.
_ this type of drive models are already under some kind of suspicion (about their factory idle3 value and about which value should be the adequate one for each usage-case); and that,
_ the WDIDLE3 version 220.127.116.11 can't be officially obtained (with relevant documentation),
then I would tend to think that performing this set of tests individually should not be taken lightly. Of course, in some cases such individual tests are not really available (e.g. when a big lot of these drives has been purchased for immediate use).
And the matter now gets a little bit trickier.
The current WDIDLE3 version 1.05, included in UBCD, runs under DOS, which depends on BIOS. Even when using LBA-48, I don't know how DOS would react to drives larger than 2TiB (approx.). It is possible that WDIDLE3 won't care about this, but we can't discard potential negative interactions. Certainly fdisk-like programs running under DOS are examples of problematic tools when talking about this limitation.
Together with the big sizes, the "4Kn Advanced Format" drives might be also a matter for consideration; not specifically for the idle3 value, but perhaps for correct identification, additional tests (DLG), partitioning, formatting...
New drives are frequently used in new systems, which are being booted in UEFI mode. No BIOS (or no CSM) means no DOS, and thus, no WDIDLE3... Which brings us to the WD5741.EXE utility running under Windows (or its equivalent for Linux), instead of releasing a DOS-based tool.
The WD5741EXE utility can be downloaded for the relevant models (such as yours) and should better be executed following the instructions posted there. The arguments should be the same as those of the WDIDLE3 tool. The user should carefully read and use the arguments so to specifically target the intended HDD(s) only.
Since the WD5741 utility runs under Windows (or its equivalent under Linux), you don't need UBCD, nor a specific bootable ISO image.
Just as with the WDIDLE3 tool, once the utility is executed it is recommended to correctly power off the system (not just to hibernate, stand-by nor reboot), so all the firmware values can take effect in the next cold boot.
Just as with any other S.M.A.R.T. value, the BIOS/UEFI and the OS should allow the S.M.A.R.T features, so to be able to reliably read the LCC value. For instance, if the S.M.A.R.T feature is set to "off" in the BIOS, then the LCC value won't be changing at every refreshed read, even if the idle3 value is set to "8 seconds" (instead of "300", or "disabled").
My humble suggestion would be for each user to search for the specific drive model in the WDC website and carefully follow the instructions posted there (for instance, for the WD5741 utility).