I am not using virtualbox, this is on my actual system. So if I understand you correctly, I should just recompile the stock qt5 version? And then what , should I remove all the export values and see what happens or.. ?

It does not matter, if you have something like virtualbox, which can set the variable to some value other than "qt5ct" it's not going to work.
So if kde or some other qt program sets this to "xcb" it's clearly visible with env, and the main reason why qt5ct does not work for you.

Which part of this suggestion below is confusing enough for you to conclude entire qt must be recompiled?

Nope. Compile qt5ct in its last version.
Remove all env.
Then test.
You'll have to choose where to add the env variable in function of the DE/WM you use.
I use .xinitrc with i3wm.

I already compiled qt5ct1.5 , removed all the env , placed it only in my .xinitrc and it still doesn't work. So.... I honestly do not know.

Relevant parts of qtdiag output;
You have "Breeze" in your screenshot, and I have no such thing here.
Maybe try some other styles in case that one is the cause of your problem, just saying.

Not sure why it would be an issue as I tried it under xfce4 and that wasn't loading breeze, but I'll bite again.... nope...changed the theme from breeze to oxygen, and restarted kde... still the same thing...

Have you fixed it yet?

Pretty sure that KDE's forcing its own theme, and the only reason why it works fine here is that I did not even install KDE.
So I guess there must be some way to force KDE to use qt5ct theme somewhere in KDE settings, but I can't really help you with that.

If it were my problem, I'd find where exactly KDE does set QT_QPA_PLATFORMTHEME=kde and change that to qt5ct.
It's most likely in "startkde" script or whatever. Possibly also in sddm config somewhere, but I use XDM so I haven't checked..

I would like to agree with you on that, but I am not so sure it is KDE - otherwise why isn't the utility functioning in xfce? It was suggested in this thread that I should try out xfce4 which also makes use of QT apps, and well the config utility still states it is not configured properly - so unless somehow KDE is overriding xfce4 , I do not know - because it is a fresh session, and I did not launch any KDE based apps under xfce4.

I think I need to start over, but now I don't know what I should remove to have a fresh start (config wise).

Funny thing with xfce is that is runs "kde services" at startup by default.
Those "kde services" only run in xfce if you have kde installed, and they could quite easily apply a simple environment variable.

Since I just use .xsession and have none of those desktop things installed, I've had no need to look into it.
On my system qt5ct-1.5 works fine, no problem. Just recompiled most recent qt5 today, and tested in blackbox wm, xfwm4 and twm.

It was stated that kde overrides .xsession, so I removed export QT_QPA_PLATFORMTHEME=qt5ct from there, it is now also removed from /etc/profile.d/qt5.sh , so essentially I am now at square one. So lets start under the premise that - well I am - under Plasma5, what would be the recommended places to put
export QT_QPA_PLATFORMTHEME=qt5ct in?

Also for the record, I only have xfce4, KDE and MWM to work with as I chose not to have blackbox/fluxbox fvwm or windowmaker installed.

Jeebizz,

why do you need qt5ct if you are running KDE? Or did I misunderstand something?

There are apps that utilize it though - clipgrab certainly does. Unless I am mistaken isn't KDE5 itself using QT5?

it is, and it applies the qt5 theme by itself. Doesn't this make qt5ct redundant?

KDE has a long history of re-inventing, so I'm certain there is a file which does the same as .xinitrc and/or .xsession, only it's named differently.

Only KDE devs can recommend this to you. Slackware just ships what they shared on kde.org.
If it were up to me, which is not, I'd tell you it's best to rewrite a /usr/bin/startkde script from scratch.
But that's just my opinion because I'm not a kde user, if you want official recommendation it's probably best to look at kde.org.