I would be interested to double check that I’m following the best practices regarding my mapping of hiking trails.
My approach is:
Should I mention in the commit message that I’ve used the provided layers and my GPX trace, or is this redundant since my trace is uploaded?
As long as you walked the trail and recorded your own GPX used for the mapping everything should be fine.
If you edit with iD, the imagery used (layers) as well as the use of a GPX track will automatically be recorded in your changeset. This is independent from uploading the GPX file, the editor records it even if you just use it locally.
What you should enter is that you did a survey cause the editor can’t know that. Manually adding the imagery used and GPX used is redundant but it does not do any harm imho. iD even offers presets for these sources.
Note that if you were navigating using that proprietary map as trail signs were missing then it seems to be unsafe to map the hiking route itself. But mapping path should be safe, as long as it was clearly visible.
If path was unmarked/unclear on the ground and you used proprietary map to follow it then I would not map such path in OSM.
Though if you planned hike and checked time/alternatives etc on trail but navigated using markers present in terrain it seems safe to map it.
Similarly if you went to Foobar Glade using proprietary map and its name is unsigned and you know it only from such proprietary map - then visiting location does not mean that you can map it.
disclaimer: this is not an official statement of any organisation, I am not a lawyer
Yes, you can map the trail (each path), however be aware that sometimes the complete trail (route relation) denomination or colour can be subject to copyright.