أحدهما One of them two - a male out of a group of two males only

إِحْداهما One of them two - a female out of a group of two females only

إِحْداهنَّ One of them  - a female out of a group of females
إِحْداهم One  (group) of many groups of famelas 

أَحَدهم One of them - a male out of a group of many males only
أَحَدهنَّ : there is not such a word in Arabic becasue there is no differnce between this word and إِحْداهم

Does -----إِحْدا refer to female and -----أَحَد to male?
That's right

i hope i could helped you and you did a great job

What's the difference? أحدهما

Do these all mean 'one of them'? I will write my guess of what they mean, but I wonder if I am wrong, because the last one does not seem to make sense.
I found them all in a dictionary, so apparently they are all true entries, but the dictionary was not clear about the exact meaning.

Here is my guess at the exact meaning. Please correct me:

أحدهما One of two males
إِحْداهما One of them two  females

إِحْداهنَّ One of them - a female out of a group of many females
إِحْداهم One of them - a female out of a group of many males or females (wrong)

أَحَدهم One of them - a male or female out of a group of many males and females
أَحَدهنَّ One of them - This one does not seem to make any sense!!!! (You are right this is wrong)A male or female out of a group of females only?? I would think if I got the meanings right this form should not exist. So I wonder....I would appreciate a clarification.

And do these truly all mean 'one of them', or do they mean something else?
Does -----إِحْدا refer to female and -----أَحَد to male?and this is right
Thanks so much!! Those terms are used to talk about the absents persons but when we talk with people who are with us we use other terms