I really don't recommend doing the trimming part in Fusion...I have no idea about the smoothing that way and what features of the ear it can crush along the way.
It's also usually computationally intensive, especially when importing raw scanned files (even more if not smoothed already) and converting quad to tspline>correcting errors>tspline to brep...It can crash easily depending on the quad counts.
The trimming, thickening and hollowing should be done with another software such as meshmixer. It's much easier then to import and play with.
You can easily close holes and repair the file with meshmixer using the inspector tool in the analysis section. Much easier than repairing while doing the quad to brep to tspline conversion in fusion. It will repair the bulk of errors and will leave you with potentially a small error count once imported in fusion, that will be easier to repair before the final tspline to brep conversion.
Once you are done with the the trimming and created the shells (that you could normally print at this stage), you could easily then convert to quads using Instant Meshes (you can control the number of quads and shapes with Instant Meshes to keep the features of your impressions as well).
Here is an example of an ear impressions that has been worked on via MM and another software for brushes as well as thickened and imported in Instant Meshes
I can then use the target vertex count to allow for more resolution for the quad count with higher subdivisions.
Here I have bumped up the vertex count and solved. Once extracting the mesh I use pure quad meshes and you can see the result
Here you can see the import in fusion
after a small repair job inside fusion that is the tspline result
The bulk of the work is actually trimming the raw impression in MM and other software to get a clean result.
It requires experience trimming in the physical realm especially if you want clean looking fp shapes and allow for enough space inside
I really don't recommend doing any kind of trimming inside fusion.