My first question is: when is Leap 15 expected to get the latest LTS version of Qt 5.9 and KDE 5.12 ?
The current version of KDE is 5.12.5 and there is the latest LTS version 5.12.6 which I could successfully test here from the additional repository.
However, regarding Qt 5.9, the current version is 5.9.4 and there is the latest version 5.9.6. I’m getting unsolvable dependency issues when trying to install version 5.9.6 from the additional repository. Both stock KDE 5.12.5 and latest LTS KDE 5.12.6 are only accepting Qt 5.9.4 as required package version.
My second question is: is there a way to test latest LTS Qt 5.9.6 on my Leap 15 (hopefully along with KDE 5.12.6)?
I may misunderstand your question, but the versions of the stable openSUSE releases (like Leap 15.0) will not get newer versions of packages during their lifetime (in the OSS repository). That is why they are called “stable”. Newer packages may be included in the next version when they have passed satisfactory testing in that version before it is frozen.
This is contrary to Tumbleweed where newer versions of packages will be in the next snapshot (also after testing of course).
In fact, I was able to update KDE 5.12 from that one. I have tested KDE 5.12.6 successfully on my Leap 15.
The issue is with installing Qt 5.9.6 and its dependency issues in relation to both KDE 5.12.5 (OSS repository) and KDE 5.12.6 (the repository you mentioned).