OK, I finally found some time to mess with this again. A new version of the installer has been uploaded to http://ftp.lyx.org/pub/lyx/bin/2.3.0/.
The problem with spellcheck was because some dictionaries were supposed to be installed by default, but there was no actual provision for including them in the package, from what I can tell. Special sauce, I guess. There were also some major problems downloading dictionaries during the installation. The URLs were simply wrong. I fixed that and simplified some of the code. Finally, there was the 'style' issue. We now install the qwindowsvistastyle.dll file, as Enrico suggested. Apparently, it is new in Qt 5.10 that these are separate files . That's why the old installer worked, I guess. It was built with 5.9.x. Anyway, I have verified that starting LyX with: LyX2.3.exe -style windows LyX2.3.exe -style windowsvista give different results. Please test, and hopefully we can finish with this next week. Riki  https://bugreports.qt.io/browse/QTBUG-65177