Like I said, the camera will set itself down to the sync speed, unless you opt for manual, in which case you will need HSS set on the flash, or you will be shooting at low shutter speed for the ambiance and movement, in which case everything will be dandy on pTTL. like my photo. You DO need a pTTL cable if you want to hold the flash at arm's length using pTTL.
If you set you aperture to f11 or similar the flash has to do a lot of work to expose properly and may not be able to reach the sensor with enough power after bouncing around the room. The card board cutout is due to the fact that the subject will be entirely lit by the flash and the room will either be dark behind or filled with ugly white flash and shadows. As I said, set the camera to be showing a pleasing amount of ambient light then use the flash to help out/fill-in/stop movement locally, like my photo

If you have a high enough ISO etc. you will still be able to bounce it.
When you turn on the flash, the camera will set the shutter to the sync speed.
I tend to use manual and find a nice darkish ambient setting, then use the flash to stop the local movement.
If people are dancing then a shutter of about 30th or even less, will help show some movement.
A diffuser per-se won't necessarily help. If it's small, it will still shine a point source onto the subject harshly picking up sweat etc.
A globe diffuser will fill the room with a little more light but if it's a big room it's likely to do nothing.
I often use a big white flexible bounce card (about A4) forget what it's called now. and fire straight up using the big reflector to light the subject, whilst also bouncing off the ceiling. But this can look a bit much in the wrong environment.
Most event togs, just blast their subjects straight-on and get away with it in an uninteresting, rabbit in the headlight sort of way. But it's safe.
A PTTL cable can have immediate desirable results, holding the flash at arm's length up high - just a lot more interesting. You can also bounce it down off the floor if it's a halloween party !

I was expecting it to be a game-changer in terms of high ISO and macro, but the K5iis is so good in the dark and retains its colour better at high iso that the difference in ISO performance is only marginal really. The z scores on retained detail of course, after you've crunched the noisy image down to normal size it certainly looks better, but I don't want to crunch a Z file down I want to use it big and for that I'm back to normal ISO range.
And for macro, I didn't consider the pixel pitch being about the same as the K5 so very high magnification, whether a flea or the moon, is actually more difficult than on the K format, particularly in view of the lack of accessories and lenses lying around for the Z.

What is a pleasant surprise is the ability to hoik up the shadows even more than the K5, so images can be under-exposed, not to avoid clipping so much as to create a softer transition to highlights. (so not strictly under-exposed, just a lot of apparently useless shadows) a God sky for example. No need for HDR.
And the images whether with the 55 or the 120mm just look like they've been taken with a 77Ltd - really 3 dimensional even stopped down with studio lights. I never see purple fringing anymore, because the lenses are actually quite slow and don't need to be wide open anyway.
If you're careless the images are often soft and disappointing, more so than a smaller format, but again, that just makes you slow down and take care and rewards you when you nail it. Showing the client a little thumbnail on the camera and then zooming and zooming and zoooming until their tiny, perfectly focused eye fills the screen is priceless
It feels lovely in the hand and at events, when people see it, nobody has any doubts that they could have got uncle Bob to do it
I knew it wouldn't perform miracles, but I kinda hoped it would, or that the images would be considerably better! but of course, only the photographer can do that and if he's good enough (not me yet) this camera will be up to the job (although that annoyingly talented guy would make an iPhone perform miracles too).

I should add that I loved the 645N film camera so much it made this decision much easier. Most of the things I have described are right there on the N. The ergonomics and wow-factor, that indefinable quality to the pictures, the careful approach and rewards therein. Clients want instant feedback of course, so I never used film commercially, but I still wish someone would ask me to.

Oh, just to add to this, I do still wonder if I should have just gone on loads of holidays with fabulous scenery, or bought all those little things like GPS modules, studio lights and batteries, broncolor parabolic brollies, etc. etc that I now can't afford. But who knows?

Some of you may be wondering how I'm getting on with the Zed (Zee) and whether I will be selling all my APS-c gear now?
The answer is just fine; I love it, haven't regretted the ridiculous outlay, and I'm looking forward to slowly reaping its rewards over several years. It isn't doing the things I really bought it for, but it is rewarding me in ways I hadn't considered, essentially a quality, from the DOF and rendering that just looks lovely.
I still haven't done it justice, but that's a good thing - lots and lots of learning and lovely surprises to come and a feeling that I took the picture, not the camera - it certainly doesn't suddenly work miracles, probably the opposite in fact; it's a tricky thing to get right.
But not only am I not selling up the Ks, I'm still coveting the K3 (which I kind of skipped with this purchase) and I think I should have another K5ii or 's'
The Zed has very specific uses for which no other camera will do, but the little Ks are beautiful things and I simply wouldn't be without them
BTW, I'm not some kleptomaniac, I do actually need two or three bodies for the event photography

Ha, didn't notice this

ha ha, nothing new there

You're right to be cautious, you're opening a can of worms there
If in doubt, put it on pTTL and bounce that bad boy off a ceiling or wall, especially if you have someone close, who might mess up the exposure.
Get as much ambient light in there as poss, by using a high ISO and(or) large (flattering) aperture, this will also help the flash to recharge and be able light your subject from a distant reflector.

I find it odd that you both like 1, I think that one is my least favourite bit over-saturated, don't like the dark corner, or the square format, or the proximity of the leading timber to the edge of the frame.
However, I like all the others for different reasons. I want the groyne to pass across the frame more tho', or to lead to something. But they're nice; worth getting wet for, certainly.

John, that's your choice, but a site able to show big images will happily show small images also.

David, thanks, but that reply has left me further in the dark.
Is the site going to display larger images or not?
There really is no need for compromise here. Mobile devices and other technical constraints do not affect the ability of a browser to display large images if you wish it to.
It seems the site's dimensions are controlled entirely by advertising requirements. But you risk losing members to sites optimised for display of images - the absolute top priority for a photography forum, and your advertisers won't be impressed by that.
It also doesn't explain why the method of clicking through (and back again) is so hopelessly clunky and why the resulting image has been reduced even tho it is displayed on its own and has no size constraints.