I got it working now. Here my discoveries:
Going back to the stable release helped. I can now flash the code using your settings with the NodeMCU board which I could not using the same settings in the staging release.
With the stable release the upload process now triggers the windows firewall during the first upload. My conclusion: javascript not finding python.exe is only an issue with the staging but not the stable version. The upload failed the first couple of times with the IDE showing a timeout. Then suddenly it started to work with no further problems (not sure what happened) - thanks to your help.
This becomes really awesome, thanks to all who contributed.
For my project I think I need the mkspiffs tool to upload my html-pages into SPIFFS. Igor added that to the staging release. So I guess I hope that one of the capable guys will fix the sync issue with NodeMCU in the next release and the javascript error for OTA too. If someone would add a feature for "Esp8266 sketch data upload" via Wifi that would be heaven...
Anyone using the terminal window via Wifi? That would be cool too...
Thanks again
Bernd