1. There are mingw64 compiled Qt dll's around on the internet.(SEH and older one version)
I think we are not same site on the internet,

I couldn't find it as long as 3 days. especially in this lazarus forum, maybe ketword or link much useful for many who looking for.
2. To build libQt4Pas.dll you need also Qt sources.
you told me that before for osx qt x64, but again not write how to, ofcourse from source.
If this is not secret and ofcource if you want, you can write article about QT installing to all os. win, linux, osx. 32 and 64 bit. I perfer forum, 'cos wiki is not updated, a few days later notworked code, wrong infos finding every million search result. But forum finding in we search and ask question and re update infos
In my country (Turkiye) we says, everything is so easy, ofcourse after learn.