I have had a Wadkin CC2000S from new in my garage for 20 years by now. Itīs also with the cc100 control. It have the drip feeding option also. That means you can create the programs on a CAM station and then feed the cnc machine just like a printer from a separate computer. Wadkin also had a special "checksum" program that did add a code to each line in the generated nc program, in order to speed up the data transfer. Programs of 20.000 -30.000 program lines were normal. Some years ago I also made small series production of chair seats like yours, with the molded upper side also machined. Also for "hand" programming I have been able to do all kind of different things. Normal G-code programming, with tool radius compensation, in different planes (x-y/x-z/y-z), running G2/3 with motion also in third axis, sub-programs, parameter programming ( I dont recommend this, because the control is too slow ), etc. And the machine has done what it has been programmed to do.
So I strongly think that it is not a problem with the CNC, but rather a problem with your post processor either not understanding your CAM program or then not always being capable of simply doing the right thing. I had the same problem at first also. It didnīt take me too long to sit down and open up the NC file on a simple text editor, and find the corrupt commandlines in the file. With that information I provided they had to re-write the postprocessor, and after that it did work.