I have downloaded fasm2 and unzipped in a linux terminal. When assembling for x86-64 a simple hello world using fasmg.x64 I get: Error: invalid argument for the format line. I use: format ELF64 executable. What should it be to assemble without error?

Use fasm2 instead of fasmg.x64. It runs the latter with the necessary headers set up.

./fasm2 hello.asm gives me:
/usr/bin/env: ‘bash\r’: No such file or directory
/usr/bin/env: use -[v]S to pass options in shebang lines

Please re-download the package. The error you get is caused by incorrect line endings in the fasm2 file ("\r\n" instead of "\n") and I briefly had it broken.

Ah!, tricky. I instead patched the fasm2 file and it now works. Is that the only place with \r's?

It's the only place that matters. The assembly source files can have any line breaks, even mixed, and fasm is still going to process them correctly.