Flashbacks.

I wouldn't ever call it "tacky," but as with anything, it has to be done well to work. I've never done it in a story under 5K, but I have an 8K story that actually has two flashback scenes in it. We'll see whether it works as I've just started subbing it to magazines.

Yes and yes.

It can be done well, but is frowned upon by many. Unless it's a critical element of the story, I would avoid using flashbacks.

I'm pretty tolerant of a lot of things that I've found many others here don't care for. I don't mind flashbacks if they're not overdone (I mean, if they're happening every other page, then your character must have PTSD ), and if they're handled properly (not too overwrought if they're the dramatic type, for example), and if they really need to be there (I guess that's a judgement call), AND if it can easily be told from the writing that that's what they are, flashbacks. On TV, you can often tell you've entered a flashback when the filming format changes--it turns to B&W, or has a soft glow, or the colors are muted or enhanced, or some such. You can't really do that in writing, so you have to find another way to make it obvious.

Mine are often italicized, though I've learned that many people around here frown on that. Again, it seems to be a matter of opinion. Sometimes I'll just do "summarized flashbacks" which are less immediate (like the difference between a memory, something you're recalling in the past, and a true flashback, something you're reliving as if it's happening NOW) but perhaps easier on the eyes. *shrug*

You know, when I think about it further, I find it odd if there's only one flashback presented in a long work such as a novel. It's the sort of technique that you expect to see at least a few times, because it illustrates how a character remembers and reacts to things, and those are things that tend to recur. Just once would kind of stand out oddly, so if there's only ONE flashback I'd probably choose a different way to present it, maybe in summarized form like I mentioned. But that's probably just me, and it is your story.

It can work, but as stated, make sure it is needed. As far as method, I use italics and switch to present tense. But the only flashbacks I've ever used are extremely short--like a short para or less.

I use them quite often and I've sold plenty of stories with them in. I generally only use one per story - more can make it seem very jumpy. I usually signal them clearly, eg 'She remembered' or 'It was all so different last year' and I don't use one until I've set up the situation and introduced the mc.