Long trill in Bach, approach from below

[peterl]

In BWV 883 (WTC book 2 no. 14 in f#) the first fugue subject ends in a long trill.  Siglind Bruhn https://www-personal.umich.edu/~siglind/wtc-ii-14.htm states that since the trill is approached from below it should begin on the main note rather than the auxiliary.  My teacher, who mostly follows the Russian school and mostly ignores historical research, agrees but can only justify it by saying it "sounds better".  Badura-Skoda's chapter on the long trill gives several cases in which the trill should start on the main note but doesn't address the approach from below at all.  I would expect that such an important case would merit discussion if it were another exception to the general rule.  His silence leads me to conclude that it's not an exception and I should start on the auxiliary.

Does anyone know of any rules or examples to help guide me here?

Once I know how to begin the trill I still have doubts about how to play it and end it.  Again, my teacher and Bruhn agree that trill and suffix should be in thirty-seconds.  At my tempo that sounds very sluggish, and if I speed it up, the work loses its devotional character.  My teacher offered as an alternative a quick trill (indeterminate number of alternations) but in that case she says the suffix must be in sixteenths!  Badura-Skoda quotes CPE Bach's condemnation of anything other than a very quick suffix.   Once again, any guidance or examples would be much appreciated.

[m2r]

When your playing Baroque music you would usually start on the auxilary note but, in the case you wouldnt for the simple reason that it actually does sound better.  If you want you can try starting on the auxilary and see how it sounds.

[quantum]

Use your ears to guide you. 

Looks like you are doing your research and digging up multiple opinions.  This is good.  The next step is to make an artistic decision.  Know that whatever you decide, there will be some who agree and some who don't.  The important thing is to make that decision in the first place and have it sound convincing. 

[keyboardclass]

Don't see the problem - A,G#,A,G#,A,G#,F#,G#,F# in 16ths.  CPE Bach says 'The average ear can always tell whether the suffix should be used'.