I reject that discussion will be inherently flawed without representation. I further submit that representation can cause flaws in discussion.

To use your thought experiment as an example, say if we are critically examining Sumerian religion and their creation myths, we may not want the Sumerians in the room even if they were readily available, since they may well take offense at us disrespecting and dissecting their beliefs. If we are discussing Sumerian religion with Sumerians and we don't want to cause them offense, we would have to tiptoe around the fact their gods well, aren't real, and subjects like "what might have inspired the Sumerians to ascribe this trait to that god" can't be discussed at all.

And to use a example more grounded in reality, a group of men can't have properly conduct a discussion on how to attract women with women in the discussion. I am well aware that this sort of discussion has pejorative connotations with how infamous the PUA community has become, but the fact is that young men do need to social spaces and groups where they can learn this, since it is skill that needs to be learned and practiced.

Or to use an even more absurd example, a group of rape victims can't have a discussion with a rapist, even though the rapist would be able to add their side of the story to the discussion.

    if we are critically examining Sumerian religion 
    and their creation myths, we may not want the Sumerians 
    in the room even if they were readily available, since 
    they may well take offense at us disrespecting and 
    dissecting their beliefs.

I would agree with this specific example.

Religion is a special case when it comes to rational discussion. It is explicitly a belief in the irrational, and is not compatible with rational thought.

When the "out group" decides to exclude the "in group" from a discussion, we should be very very sure that there's some highly specific reason why the "in group" is simply incapable of rational discussion.

For example, we exclude my car from discussions about his medical care because he is a cat and he can't speak or understand medicine. If we do that with people, we need to be very careful.

    And to use a example more grounded in reality, 
    a group of men can't have properly conduct a 
    discussion on how to attract women with women 
    in the discussion. 

This is completely opposite to my experiences.

I certainly think it's healthy and good for men to discuss sex, attraction, etc, without women as well. Heterosexual men are a part of the group that experiences dating and sex with women. (I wonder how many hours of my life I've spent on this? Thousands? Tens of thousands?)

This is markedly different than, say, a panel of men discussing/deciding things for women, i.e. a panel full of men deciding what women can and cannot do with their bodies.

    a group of rape victims can't have a discussion 
    with a rapist, even though the rapist would be 
    able to add their side of the story to the discussion. 

I agree with caveats. (Some victims find power and closure by confronting their rapists, etc.)

To generalize this specific example into something broadly applicable, the reason why this example works is because in this case the rapist has done something highly transgressive - essentially, they have broken anything that might reasonably be considered a social contract - and it would certainly be reasonable for a rape victim to find it highly upsetting to see their rapist, much less listen to them.

So again, I would say the validity of excluding a person from a discussion relevant to them would highly depend upon some explicit evidence or reasoning that productive discussion simply cannot occur if they are a part of it.

This would not apply to, say, a room full of white people deciding things about the Black experience.