mare, fons, and ad...
We should not have put mare into that quiz, nor fons so for both quizzes I would merely choose another word... perhaps cor, cordis for quiz 21 and iter, itineris for quiz 22.
Now to ad... one of the definitions of at is "at" and we neglected to include that in the instruction on ad..., good catch!... it reminded me of the use of a similar preposition in Hebrew. The Lamed in Hebrew is used as a kind of catch-all preposition but literally can be rendered "the son that is to/towards you" instead of the better English vernacular "your son." The connection between to and at seems related even there... since there is a motion/proximity idea inside of ad.
You mentioned the motion towards idea being ablative but there also that motion towards sense or what I call "referential location" sense in acc. prepositions... but at the end of the day... ad takes the acc. and that's that.