You should try two simple gdal tools: gdaltindex and gdalbuildvrt.
To create the mosaic, since your rasters are georreferenced, you just need:
gdalbuildvrt mosaic.vrt *.tif
QGIS will open this mosaic.vrt raster.
If you want to create the grid around your rasters, you can use gdaltindex.
gdaltindex mosaic.shp *.tif
Open both the mosaic.vrt and mosaic....
According to this forum
The match option performs a linear regression (based on the values of
all overlapping cells) in order to fit the values of the grid in
process to those that have already been added to the mosaic.
From my personal experience, the Fit option, allow you to center the pixel on the center of the cells or to one of its corners (can'...