Apart from the cold local heating being, in my opinion, the catch/hook, it is unclear to me where you get the certainty that your (still unknown, booo other hook) neighbor will still be on the previously assumed side after subdividing into 7 instead of 6 plots. Furthermore, when I read "church" and "Volksbank," I could imagine that you are actually weighing a "pear" versus an "apple" option: the Volksbank usually sells plots for purchase, whereas the church is known for its preference for leasehold rights.
Good coordination with the half-neighbor is T.H.E. K.E.Y.!!! to building a semi-detached house that is not under a bad star (see Goalkeeper thread). If you want to build more house depth (what makes you even fear that the neighbor wanted to build shorter/smaller?), then I see no problem with that for a flat roof. You don’t need to worry about a house width of about 6.5 meters: the standard models from the "usual suspects" have 6 meters width (and 12.50 m depth, come with a 35° gable roof at about 140/145 sqm, and the stepped floor variant is not significantly different). An example currently discussed in the forum (although the thread is about the insolvency of the general contractor) can be found in Ratingen (Felderhof quarter). There, such models are mixed "rows" of gable-roofed and "rows" of stepped floor variants (Delight / Glow). Your half a meter more width unfortunately gives you little opportunity to noticeably broaden the individual framework of the floor plan layout, even if you are formally not confined by the housing type proposal’s corset: look at the models of the "usual suspects" and you will quickly realize that they "resemble" each other like GDB mineral water bottles. You also cannot get around the elongated, single-flight staircase with inward and outward turns even with the extra half meter of width. Where you have more possibilities is the eased area of entrance / WC / cloakroom, otherwise it will not be far to escape the "family resemblance" of the species. These objects are all very clear "middle houses" (Yvonne prefers to say "starter properties") for young families with a holding period until the offspring become teenagers.