Taking this complex sentence piece by piece:
Results, I am certain, are so thorough that...
I'm not entirely sure how to interpret "Results", but it seems more likely to be a noun (plural, to agree with "are") than a verb. Going off that, there is an implied "The" before "results". But whose results? The results of the speaker's account (since this line is right after they say they will tell us of what happened) or the results of the government's actions (since that is what could keep the people safe)?
Anyway, some results are "so thorough", or so complete, that - something. The speaker is certain that the results are this thorough (thus the comma-separated clause "I am certain" is put right before "thorough"). What is the speaker so certain that the thoroughness of the results will lead to?
...that no public harm save a shock of repulsion could ever accrue from...
Here "save" is used to mean "except". Then "public harm" means "harm to the general public", "shock of repulsion" means an acute disturbance consisting of a feeling of repulsion/repugnance, and "accrue" can be simply translated as "come" or "result"
So this excerpt means "...that no harm to the public except an acute, sudden feeling of repulsion could come from...". The speaker is certain that no harm except this repulsion could come from - what?
...from a hinting of what was found by those horrified raiders at Innsmouth.
Now "hinting" means telling hints, or little bits of information, and "a hinting" means an act of hinting. What's being hinting is "what was found" - the things/creatures/scenes that were found - "by those horrified raiders at Innsmouth".
Earlier in the story it was mentioned that the government raided Innsmouth and then destroyed the area. When "liberal organisations" made complaints (presumably about the lack of transparency and perceived overreach) representatives were taken "on trips to certain camps and prisons" and then "became surprisingly passive and reticent". This implies that whatever they were shown was so horrifying that they decided it wouldn't be a good idea to release the information, or badger the government to do so. The raiders who saw everything firsthand (and not under the controlled conditions of a visit) must have been even more horrified at what they saw "at Innsmouth".
The speaker is certain that no harm to the public except an acute, sudden feeling of repulsion could come from giving small bits of information about whatever the Innsmouth government raiders found.