Here you go. Pass this on to the trainee. Maybe it will help.
From front to back.
At the left I’ve draw a circle for a Follow Me path. I also drew the profile for Follow Me operation so that it is centered on the circle and the red axis. Make sure to always draw circles dragging out the radius on axis. That helps to eliminate a number of problems which appear in the original and your fixed file. I added the bit on the right end of the profile for the two splines that extend beyond the end of the tube. And to the left of the profile, I create what I’ll call the broaches to cut the space between the splines. Below the profile is a line I drew to use for scaling. The rest of the process needs to be done at a larger scale to avoid the tiny face issue which is probably what made much of the model disappear for you when you ran Solid Solver. To scale the model up, measure that line with the Tape Measure tool making sure to click at each end. Then type the desired dimension. I scaled it up to 1000mm long.
Select the path circle, get Follow Me and click on the profile. Correct the face orientation if needed so all exposed faces are white front faces. I moved the “broaches” onto the end of the turned part of the model.
Use Paush/Pull to push in the waste leaving the two longer splines. The first Push/Pull operation will stop at the end of the larger diameter portion.
Push/Pull the “broaches” into the opposite end of the interior. Erase a few unneeded edges from the ends of the short splines on the right end of the model. Scale it back down to the correct size using the method described for scaling up and make it a group or component. When I made the component it was solid right away. That would indicate that there are no holes or other problems with it. Solid Inspector2 reports everything is shiny.
test.skp (788.1 KB)