To summarize the solutions:
Multiple bands in outputs when using BandSelect
This is the expected behaviour of BandSelect. It outputs masks and flags as extra bands.
Use Subset instead.
Example:
<node id="Subset_OZA">
<operator>Subset</operator>
<sources><source>Reproject_OLCI</source></sources>
<parameters>
<sourceBands>OZA</sourceBands>
<copyMetadata>false</copyMetadata>
</parameters>
</node>
NaN in outputs after subset
Using pixel geocoding replaces some pixels by NaNs. This appears only when using the option:
" -Ds3tbx.reader.olci.pixelGeoCoding=true" in gpt
Error when reprojecting in EPSG 3413 in SNAP’s graphbuilder
Under investigation.