[Koir]

Originally Posted by YuriTokoro

So, in this case, “its” means “the role individuals like you play in keeping JapanForum a successful site on the internet for cultural exchange.”
(However, this seems to be too long.....)

The meaning of "it's" in this context is not interchangeable with the longer phrase for the reason you state (too long). In this sentence, I have essentially restated the subject (individuals like you) three times to provide emphasis and an indirect reference to you personally. I did this because I was unsure how such a direct form of address would be received, and ended up overwriting the sentence and causing confusion.

“not to mention its continued success” = “not to mention the role individuals’ success”.
Is this right?

This is the collective pronoun "its", which is different from "it's" (or it is). "Its" is a word used in this context to refer to an object that is not a person (JapanForum), and functions as a subject for "continued success" to refer to without restating the subject by name a second time.

This is a very easy mistake to make, even for native English speakers.

You mean “not to mention that we are doing very well in this forum”, don’t you?

Essentially, yes. At least in this thread, which is a very welcome part of the forum that keeps me interested and challenged.

They require slightly different contexts and sentence structures?
So, that’s not to say that I can say any of them instead of “not to mention”, right?
“…X, much less Y.”
“…X, let alone Y.”
“…X, to say nothing of Y,”

What are the differences among them?

"...X, much less Y." - in this case, the phrases or concepts of X and Y are thought of as equal worth to the speaker. They may be related in a way that one concept is a result of the other, and so cannot be considered separately.

"...X, let alone Y." - in this case, X is given more emphasis or "weight" by the speaker as a separate concept. "Y" is a concept that has almost the same emphasis as "X", but can be separate from "X". Concept "Y" 's presence in the speaker's statement functions as an addition to "X" mainly to show that "Y" is not specifically needed to understand the statement. It is there to add weight or emphasis to the statement, should the reader need additional information to understand the sentence's concepts.

"...X, to say nothing of Y." - in this case, X and Y are completely separate concepts that have equal emphasis or weight to the speaker. Concept "Y" appears as a separate concept that could be in its own sentence, but making a separate sentence would duplicate concepts or context.

If I say “It's helpful individuals such as you that provide the reason this forum was made, to say nothing of its continued success.”, is this strange?

It's not strange, Yuri. In fact, it's exactly right!

I apologies, too.
My inapprehension seems to be too terrible.
Koir, Thanks.

You're welcome, Yuri. I hope my explanations can be of some use.