Unable to add dyndbg to command line

This is correct. The essential thing to understand is the kernel's command-line uses space as the separator between options so:

option1=1 option2=2 option3=3
option1=1 option2="2" option3="3 or 4"
option1=1 "option2=2" "option3=3 or 4"

are all identical.

What you are seeing in /proc/cmdline is the *kernel* writing out each option and, when an option contains a space, it surrounds the option with double-quotes.

Consider that the "=" symbol has no special meaning in the context of the command line although individual modules *may* use it to separate key=value options themselves.

The reason your "dyndbg=module e1000" does not work is because dyndbg=... is only processed at boot time and therefore only affects *built-in* modules. e1000 is a dynamically loaded module not a built-in.

What you need is the module.dyndbg=... form as in:

"e1000.dyndbg=+p"

See especially line 259, 291 of:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/admin-guide/dynamic-debug-howto.rst

This is not correct. The advice applies only to enabling debug messages in modules, but how does one enable debug messages kernel image? For example one could want to see debug messages from kernel/module.c which is not in any module, and the kernel parameter would be:

  dyndbg="file kernel/module.c +p"

but grub turns that into

  "dyndbg=file kernel/module.c +p"

which kernel fails to parse.

There are other reports on the web:
http://savannah.gnu.org/bugs/?44580
https://access.redhat.com/solutions/1152603
https://lists.ozlabs.org/pipermail/petitboot/2016-September/000477.html