Joanne,
Good for you for wanting to bring this to your mother's elders/ms attention.
Path does make some good points, however. Perhaps you should put in a sentence, or so, stressing that she doesn't know that you are writing the letter, and that these are your personal observations -- that she, personally, doesn't 'complain' to you, etc.
I'm concerned that the part about her being 'critical' of how she is treated, will just make things worse for her (even if her feelings are expressed only to you.)
I'm glad your mom continues her relationship with you, despite their shunning mandates. But, if they view her as 'complaining' to an 'apostate', even this might be called into question.
Do you know of any one elder in her congregation that you view as an individual with compassion? If so, maybe you can send the letter to him.
I've known of many similar situations...it always sickens and saddens me...sigh.