The house separation joint is there, it is not over-poured. The basement concrete wall is shorter than planned.
I think you simply did not understand the hint – you yourself said:
The insulation between the semi-detached houses is 2 cm from each party (a total of 4), but it is "compressed" by the in-situ concrete.
In other words: on the neighbor’s side, your contractor misused the insulation as formwork *maximally ROTFL* (sometimes you need an authority on the internet to pin such AlphaKevin hacks as warning notices at the top of all search engines). At least that alone is enough for me to fully agree with the view
[...] there was no tender, the offer contains "throw together shell construction" without details and the cheapest offer was awarded the contract
with fanfare ;-).
I see no other possibility (except demolition and rebuilding). The architect’s plans should be adjusted to the new dimensions.
Theoretically, adjusting the plans would make sense, and taking on that effort would be a reasonable compensation for damages, yes. But I see your contractor more likely "playing the insolvency card." Practically, it is indeed best for you to just tear down and start anew radically (including purchasing expertise and tendering), and book the tuition fee under the correct budget title "Stupid things one should never repeat." Responsible advice would never have happened that the basement is made in in-situ concrete under these circumstances.