I'm inclined to agree with @dwn. The relative major here isn't particularly tonicised: it is just emphasised a bit by melodic motion through the upper auxiliary, and a slowdown of harmonic rhythm. It is otherwise just a natural conclusion to the preceding sequence. It's the next 2 bars that start to look a bit more like a conventional cadence, only Bach reinterprets D in the second half of the 2nd bar as the seventh of a dominant 7th on E and moves to A, and thence to E, using the figuration of the last bar of your example.
The nature of the piece is very much "toccata": Bach isn't going to make any cadences before the end that aren't undercut in some fashion, and the easiest way to end a sequence without stopping is just to change the figuration (and harmonic rhythm in this case), and start a new sequence.
As for the WTC example, I think it is proper to say that the entire bass figuration after the tie is an anticipation. Look at the outline it takes up to and including the low F: E-A-C-F. Then take a look at how he delays the resolution of the suspension over the F to emphasise the IV7 sonority.
The A in specific is an anticipation of a note that isn't there: if he had continued the melodic sequence (up a 3rd, down a 5th) in the bass, the note immediately following F would have been A. That, however, wouldn't have confirmed F as the root as well as C does, and wouldn't have allowed the ascending scale segment to set up ii6 (which is anticipated on the 4th beat of the bar). The setup in the second half of the bar, as he has it, can be read as IV7, vi6, ii, vi 6-4 (and thence ii6), and also more simply as an elaboration with passing notes of IV7 - the ambiguity, I think, is deliberate, but would have been somewhat broken if he used A in the bass after the low F. There's even a little ambiguity as to whether the D [semiquaver] in the descant [in the figuration] on the 4th* [3rd] beat is a neighbouring* [passing] tone or a resolution of the suspension, which throws the real weight of resolution onto the start of the next bar* [beat].
Now here's the interesting notion: what happens immediately after the low F is suggesting a harmonic continuation of the "up a 3rd, down a fifth" movement with its suggestion of vi and ii. Notice too that there is a pattern working through the entire bar, that of anticipating the following harmony by a variety of means. I haven't analysed the entire prelude, but I would not be surprised to see that pattern showing up a lot.
The A in question is thus something of an anticipation of a "pensato", and yeah, I think Bach was sneaky enough to do something like that. I don't think there's any question that the harmony in the first half of the measure is the dominant 7th on C: the A in question doesn't have enough weight (unprepared, on the 4th [semi]quaver of the bar* [2nd beat]) to become a root.
The main point to take away from all this, I think, is that Baroque composers, including Bach, didn't necessarily see all the voice leading as a figuring of the bass: a lot of notes (even in the bass) would be considered non-harmonic tones, and those could be used both freely and motivically, as was done here. The A in question can be seen as a kind of nota cambiata that helps to trace a broken chord on F and anticipates the following harmony through melodic means.
Edit: The last sentence of the fourth paragraph (2nd after the break) should read "There's even a little ambiguity as to whether the D semiquaver in the descant in the figuration on the 3rd beat is a passing tone or a resolution of the suspension, which throws the real weight of resolution onto the start of the next beat."
The last sentence of the 6th paragraph should read "I don't think there's any question that the harmony in the first half of the measure is the dominant 7th on C: the A in question doesn't have enough weight (unprepared, on the 4th semiquaver of the 2nd beat) to become a root."
This is the problem of doing an analysis of this sort in the wee hours of the morning: I become innumerate. <wry grin>