Good research. Your first reference does a good job of pointing out the subtle distinctions. In particular, it points out that "insidious" is
- "(... neutral, but the word is rarely used except in a negative sense.)"
The Oxford dictionary does define them similarly, but not identically:
for "insidious", notice that it mentions both the gradual nature and the harmful effect together.
For "pernicious", it mentions first the harmful effect and then says "especially" about the gradualness.
So according to Oxford Dictionaries:
"insidious" things always come on gradually and always have harmful effects.
"pernicious" things are always harmful**, and often (but not necessarily) come on gradually.
So you could use either word for things that are very hamful and come on gradually or subtly.
However, for something that is only, say, annoying (like that song you just can't get out of your head) you could say it is "insidious", but not "pernicious". It insinuates itself, not innocently, but innocuously.