I have found this topic as I was searching for "forward merge".
The only thing I would add is that if you had 2D things manually drawn in Sections, Elevations, etc., those will not transfer with a Forward Merge. However, you can copy-paste these between the two files.
Also, Views will not be merged so you would have to recreate them, and probably some other settings won't transfer either, only the geometry.
However, this is still a good solution, especially compared to having to work with a crashing, buggy file.
Also, ARCHICAD 17 and later have the "Open & Repair File" checkbox in the Open File Dialog, so that would be the first thing to try. If that does not repair the corrupt file, then the forward merge method can come.
Loving Archicad since 1995 - Find Archicad Tips at x.com/laszlonagy
AMD Ryzen9 5900X CPU, 64 GB RAM 3600 MHz, Nvidia GTX 1060 6GB, 500 GB NVMe SSD
2x28" (2560x1440), Windows 10 PRO ENG, Ac20-Ac28