I don't know if this will help in the discussion but here is what I can say about it
The echo is an inherent quality of having an ancient soul. It's purpose is not specifically for warding against tempering, but offers resistance by virtue of sheer power/strength. In ShB Emet mentions that Zodiark tempered them while implying that even the strongest can be tempered if the primal is strong enough. This clearly means, the ancients has some level of resistance based on their power. Echo-bearing-sundered people or resonant-bearing people could also have that kind of resistance, albeit weaker and the strength of it could entirely be dependent on how strong the person is wielding that echo/resonant. Zenos and Fordola are quite strong enough, I think to resist some primals. Same with WoL, as seen during the HW. We know that echo is manifested differently to differently people based on the power/personality of their unsundered soul, but common to all is the side-effect of being resistant to tempering.
The Blessing of light is entirely from Venat/Hydelyn. Venat mentions that a traveler's ward was placed on the WoL which is a magick specifically made to "Prevent" tempering. So in contrast to echo which provides resistance, traveler's ward provides protection. I hope the distinction is clear. As to the Blessing of Light, it is not clear whether it only means traveler's ward. Venat mentions that there are other protective magicks, but the traveler's ward is one specifically made to prevent tempering. This could mean that the blessing could be collection of all magicks that Hydelyn/Venat imparts to the WoL, one of which is the traveler's ward.
We also know that when someone awakens the echo, an "automated" message is sent directly to that person which enables them to see the meteor shower and the 'hear, feel, think'. This, I believe is way of making sure Hydelyn initiates the first connection with the echo-bearer and which they are given a test of character to become WoL. A test in which if they pass, they will be granted the full power of the Blessing of Light. In our WoL case, the test is represented by all the crystals that we lit during the ARR story. This could be a way for Venat to get as many WoL as possible to serve her battle calamities or the final days.