Apologies for another too-long post from me, and apologies again if I've misunderstood or taken your question out of context.
How does one indicate, without giving offence, that an argument from authority is weak?
Doing that (without giving offence) can be difficult.
I think the only time I suggested that a Pali text might be weak is when I did that by quoting other (expert) people who said so (i.e. I referenced their long published paper which compared a Pali sutta to a Chinese version).
And depending on the question, an argument from authority might be on-topic: if the question is "what is the Buddhist doctrine about X?" and the Pali texts are Buddhist doctrine, then so long as the quoted Pali is on the subject of X then it seems like a reasonable answer.
Or is this a case of offering a different kind of answer and letting the public decide?
I like to distinguish between "negative criticism" and "constructive or prescriptive criticism."
For example mere (negative) criticism might be a comment like, "that's a bad answer, it's just an argument from authority" or "that's a bad answer, the Pali texts aren't authoritative."
A better criticism is at least informative, adding or leading to further information: "I see you quoted the X sutta. Given that you're interested in it, you might lead to read paper Y by researcher Z who argues that this sutta is often misinterpreted" (or "... a forgery", or "... written by someone who doesn't know what they're talking about", or whatever the criticism is). In that case you're informing as well as criticizing, offering to expand rather than simply to destroy someone's view.
Dare I suggest you consider "right speech" in deciding whether and how to criticize? Do you think your criticism will be welcome, timely, true, useful, etc.?
I don't often downvote (about 7% of my votes are downvotes), but I will downvote if I think that an answer is misleading or worse than useless; and if I downvote, then I also try to post a critical comment (to explain the downvote or to give the author a chance to improve their answer). If my leaving a comment leads to an argument with author and apparent anger and getting personal (complaining about the person instead of discussing the answer) then I may avoid posting comments to that person in future (i.e. I might do anonymous downvotes without leaving a comment).
An anonymous downvote is the simplest and most friction-free (albeit also the most meaningless or least informative) way to criticize.
Before you comment, do you consider whether you expect that the author will be able to improve this answer? This answer for example, I thought it was obvious (because he said, "I don't have personal knowledge of this branch of Buddhism, but I've made use of a search engine to find these sources") that he couldn't easily alter his answer, so this one was a take-it-or-leave-it as-is answer.
I try to be conscious that we're writing for two audiences, i.e. the current users of the site and future readers. I don't want to antagonize current users for the sake of "the public".
And "yes" to offering a different answer, if you want to write something other than what other people have written.
What's better than argument from authority? Possibly argument from experience? That's not always an infallible guide either though (e.g. because there's also lack of experience), and neither is logic (e.g. because it depends on what axioms you choose and on how you simplify reality to describe it).
So IMO answers do "make a point", and hopefully the points they make are true, even when not the whole truth. And one of the points is apparently that the Pali text says X. A fuller answer might explain how X is relevant to the question (but that better might be the enemy of the good).
One more thing, I do think we ought to be reserved about criticizing other people's beliefs. Look at an answer like this one for example or this one -- if a single user or a single group of user becomes relatively abrasive about complaining about other people's views (no matter really whether they're a minority or a majority view) then that may make other people averse to participating at all. This is meant to be a general forum and people should be allowed to participate at present diverse views ("Mahayana says this..." and "Theravada says this..." etc.); but negative views ("Mahayana is a heresy" and "Theravada is a forgery" etc.) are difficult.
the only reason to consider it true is religious belief
Maybe you don't have a religious belief but if "secular Buddhism" is one form (not the only form) of Buddhism then I hope it will co-exist on (and not try to dominate) this site.
Perhaps I should point out this topic: Answers vs Advice
It isn't necessarily anyone's job, on this site, to criticize other people's religious beliefs.
I suspect that the StackExchange model is primarily to post a good answer, and to upvote other answers you agree with (or to upvote instead of posting if other answers have already said what you would have wanted to).