Did you use any professional recordings as guides when developing your interpretation of BWV 914? I'm drawing heavily from Gould - I like his recording of 914 better than any of the others I've heard, but I'm not among those who consider Gould the be-all, end-all of Bach. (For instance, I tend to prefer Schiff and sometimes Argerich for the WTC.)
I love Gould but wasn't totally convinced by his performance of 914 - I found it a little detached (emotionally), unusually for him. There's an amazing video of Sokolov on youtube playing most of it, I love his interpretation. But also worth looking at other student recitals on youtube, they all provided interesting elements even if the overall performances weren't as good as the better known pianists.
One thing I found super useful was to record myself - I didn't do this until 2 weeks before the exam and freaked out a bit because it sounded terrible (after listening to the masters for two months) but I quickly adapted my playing to sound better to the audience. The DipABRSM (and above) is supposed to be a full professional recital, so I think this is a really important thing to do, and I'm doing it right from the start on my LRSM pieces.
Another really useful thing that I didn't discover until 2 weeks before the exam was researching and writing the programme notes. I learnt so much about Bach, keyboards, composition and the piece that I looked at it differently after I wrote the notes. Again I've started researching the pieces up front for my LRSM pieces. Particularly with the Bach 914, the fact that the notation we have is quite second-hand (comes from students/performers copies and are quite inconsistent), the fact that performers of the time were familiar with the style of playing, ornamentation, improvisation and notation of the composers meant that a lot went unsaid in the written music, the early Italian influence to Bach at the time it was written, that it was probably written for 2-manual harpsichord, all that influences how you approach learning and interpreting the piece.
Another question for you regarding 914: did you play the upper mordents in the Fugato and Fugue movements? If so, did you play them as upper mordents (primary-secondary-primary) or as short Baroque trills (seconday-primary-secondary-primary)? The edition I'm using recommends playing them as upper mordents in the Fugato, which seems to make sense musically. It doesn't make a specific recommendation for the Fugue, but Gould seems to play them as short trills in that movement, and short trills seem to make sense and fall under the hand better than upper mordents.
Given that the ornamentation varies widely across the different editions, and also across performances from established pianists, and that there's no evidence it came from Bach himself, and that the piece is highly improvisatory in nature, I ended up playing around with the ornamentation quite freely which was quite fun to do. I dialled it back a bit for the exam but there's lots of places you can have fun improvising the ornamentation in the first movement. I ended up using mostly the Peters urtext edition which only has one single mordent notated in the second movement but I added the odd twiddle (mordent) in to provide some harmonic continuity or just a little interest here and there. I didn't dare in the exam but I quite like doing an 'inverted mordent' (p,p+1,p) in bars 20/21 in the right hand on the dotted-C and dotted-B, even though that ornamentation apparently didn't exist in Bach's time - I just think it sounds right there. Is that what you mean by 'upper mordent' - p,p+1,p as in the primary note, upper note and primary note? As far as I could find in my research that doesn't exist in Bach's time - there is either the mordent (p,p-1,p) or the trill (p+1,p,p+1,p). There's a trill written in Bar 51 that I couldn't pull off cleanly so I just played a mordent - it sounds better for my playing. In my edition in bar 84 the F# has a short trill written and in bar 85 a longer trill, but from what I read a trill is a trill - how many twiddles you do is apparently more correctly determined by the length of the note it refers to and the will of the performer rather than what one edition says. Gould was so incredible technically he could get away with more ornate ornamentation and still sound good, I prefer to dial it back so whatever I play still sounds natural and in control.
It's quite tricky to play the fugues well, worth taking your time as it eventually comes to you and then feels great. I've played it a few times since my exam and it doesn't feel like an exam piece. I absolutely love playing it, immensely satisfying - it's a friend for life now