Slicer Issue or STL model issue...not sure.

[fred_dot_u]

I'm not hopeful about your results. Once it was "flatted" and repaired by PrusaSlicer and exported, I ran the preview and there are faces where I don't think there should be faces. In Meshmixer, the automatic repair ripped big chunks from the model. Now that I've typed this, I remembered that Netfabb has an online repair service (free!) so I pumped the model into that. It appears that all the flaws have been repaired with great results. File attached: