The chord? The lowest note with a finger more arched than usual; the finger playing the higher note must briefly curl and press the A string t the right rather than down on the fingerboard. The wrist can swing under the neck as soon as possible. Uncomfortable!
You can also bring down the 3rd finger for the first high C of the measure: less stretch.
In the context of this (or any other) courante the Mazas etudes quoted above are indeed good preparation. There are similar passages in other Bach works (double concerto, very first solo for example) and sometimes the context is different and you need to play a continuing melody across such a large interval. In that case the difficulty is less easy to solve. There is a sonata by Vivaldi (op. 2/2 in A) where such a passage occurs. When we were teenagers a group of us had decided to prepare some music and perform it in old folks homes and similar venues where people would be enjoying any kind of entertainment. Among other pieces I played this sonata (a piece well worth it BTW, much more inspired than the a-minor concerto). Everybody in the group nagged me about that passage, telling me that I could not possible leave a "hole" in the music. So I was forced to try and figure it out. I can't tell you exactly how I did it or how it should be done but I could do it sufficiently to at least pass in the eyes of my colleagues.
My brother liked to play it (on the cello) when we were both living at home and he played the one-slur-per-measure bowing that was commonly used in the 60s and 70s but that the HIP crowd has apparently since done away with. I still think that musically it is the best solution (because it presents the music as melody rather than chopping it up in 16ths). I may be biased by being used to it though.
I like slurring in fours, but even if I slur only three notes I detach the remaining seven smoothly, with a more marked détaché whenever the arpeggios are interrupted by step-wise motifs.
I certainly avoid rubato, which destroys the flow, and ruins the contrast between murmuring arpeggios and dynamic scales.
I love the way Bach has a bit of fun in the middle (as so often) before climbing up to the the final "hanging garden" effect of the last measures.
I had a piano teacher when I was a kid who though she was a very expressive player or something and she rotated her torso around on her hips while she played, like a slow-moving top. One day she overdid it and her salt-and-pepper wig fell off.
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For the chord, you have to finger that 1-0-2 and you're just going to have to get your elbow far enough your instrument "by any means necessary" as Malcolm X used to say, so that you can stop the G# without stopping the B. Again, watch and listen to cellists so that you can get an idea how they solve this problem and how much time they spend on the G# as well.