I'd agree with your thoughts.
Say for an ale, the first lot of shyte into the cone will be trub: some hot break and the cold break. I find that the cold break tends to get "pickled" and drops out fairly soon. So you can rotate the racking arm down into the cone, run off much of the break, thus getting rid of most of that before FG, then rotate back up into the beer.
Then during fermentation over the next few days, yeast will rain down into the cone, filling it up.
However at this point, (edit: particularly if doing a lager) if you wish to collect the yeast for subsequent brews, you need to decide whether you are going collect the yeast at FG then proceed to cold crashing, or cold crash and collect the yeast at the end of lagering or cold conditioning.
If kegging straight away without saving yeast, you can just position racking arm, hopefully, above yeast cake level and go about your business then dispose of the yeast.
However if you want to harvest yeast it's probably the best time to do so, because if you leave the yeast there then crash chill, you could end up with shocked yeast and other issues.
So if cold crashing and harvesting, I'd rotate down into the yeast cake at FG before chilling, then draw off yeast for reculturing, then lager the beer.
I'd speculate that if you don't have the long racking arm then, if you have disposed of the trub earlier, then the short racking arm as supplied should only be picking up yeast. Any remaining trub would be sitting in the bottom of the cone out of the reach of the racking arm.
