How can we convert an entire QGS Project file with multi-geometry shapefiles into a single KML file with styles from QGIS?
When you convert Shapefiles into KML files and uploading them into google maps the style will change. It is pretty easy to change the google default style into the style you need. About these multi-geometry shapefiles there is a possibility to merge them into one shapefile. They all need to be the same file type, you can merge to polygons layers, but you cannot merge polygon with line or point layer. Also the data in the attribute tables need to be similar, if they don't have the same columns it will not work.
Take care to the fact that this new layer is only exits in your project, if you will not save it as a new layer and accidentally erase the project this new layer will be lost.