These all say "pretty much the same thing".
From the list of examples, you have both "honest" and "descriptive" which have wildly different meanings, intents and connotations. These are not synonyms.
The main problem with the question you are asking is that you have an artificially restricted situation. The variations should include variations such as "less dishonest" or "a fair way to say it" and you can scope out even further to modifications such as:
Due to my low reputation on SE, I was tentative (read: fearful) in posting to meta.SE.
At the end of the day, you simply have a meaning to convey and English provides many different paths to get to that particular meaning. The only qualification that matches your original examples is "adjective that helps compare 'tentative' and 'fearful'". The specific comparisons are rather different (e.g. "honest" versus "descriptive") and there isn't anything about this particular construction that couldn't be replaced by an entirely different phrase (e.g. "read:")
Thus, the only interesting question I see is how to describe this particular meaning. You offer:
[Main clause stating some condition with an understated tone] (well, perhaps [emphatic tone word] is [what goes here is allowed more variation than normal])
What is really happening is a deliberate shift in connotation that forces the reader to update their understanding of the situation to match the author's. In this case, you are providing three key pieces of information:
- You are fearful
- You don't want to admit you are fearful
- You are willing to admit you don't want to admit you are fearful because this playfully solves the problem from (2)
Why (3) works is a question of rhetoric and probably outside of the scope for this site. But the fundamental point is that you need to convey (3) and an extremely efficient way to do that is to use a word that allows you to shift from a subtle connotation to a self-depreciating connotation. The list of words, phrases or conventions that accomplish this is massive. While you can certainly find large batches of synonyms on the list, the list is not restricted to one set of synonyms.
To directly answer your questions:
Would these just be called synonyms? Distant synonyms? Synonymous?
No, they would not be called synonyms. The closest relevant term I can think of is "self-aware, self-depreciating rhetorical device" and that describes the function of the entire clause. I don't know if there is a more technical term to describe this.
And also, is the idiomatic construction allowing a wider semantic distance for the blank?
Yes, the construction is extremely flexible and has many different variants. Some of the most common:
... tentative (read: fearful) ...
... tentative (read as: fearful) ...
... tentative (*cough* fearful *cough*) ...
... tentative (er, fearful [X]) ...
... tentative (okay, fearful [X]) ...
... tentative (well, fearful [X]) ...
[X] could be anything from your list or similar lists or, sometimes, simply nothing at all. The list of "er/okay/well" options is also much larger. And you will occasionally see the positions of "tentative" and "fearful" swapped:
... fearful (uh, tentative -- yeah, tentative is a better word for it) ...