OP here....in answer to a few questions:
No, she doesn't volunteer. We have a minimum volunteer requirement, but she gets her older daughter to do it so she won't have to. In fact, we had a minimum age or if you were under the age, we had a working-with-a-parent-for-several-hours-before-you-can-work-alone requirement. She was informed of it (by DH) and kind of blew him off. Then she tried to have her kid work anyway, and got all irate with DH when he called her on it. (They'd also 'volunteered' the daughter to work for other people if they paid her, so I also think they were upset that the daughter wouldn't be able to make some money.

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She's divorced from the kids' dad and lets everyone know how awful he is.
All of the coaches are volunteers, but we still do charge fees to play. We have a facility to maintain (with taxes!), officials to pay, equipment to buy, etc. When she complains, that is one thing that we hear a lot of....how much she pays, yadda yadda yadda. I think the next time I personally hear this, I'm going to tell her that with all of the hours that we put in volunteering, WE still pay full price too!
One of her big things is that if her kid is sitting too much or isn't playing key positions frequently, she'll fire off a scathing letter to the coach. She did this with the current coach. I'd talked to a previous coach, and she did the same thing with him (he actually volunteered this info to me when we weren't even talking about her at all, so I know she made the same impression on him.) Luckily, this current coach had very good records of when/where he played each kid and how often each kid sat out. He was able to confront most of her 'assumptions' with hard facts to refute that what she was saying was simply not true. One of her beefs was that the kid never got a chance to play a certain position (actually, two positions, and they're the most difficult positions and most highly skilled positions to play). The fact was that the entire team was asked at the beginning of the season who wanted to do that, and he never expressed an interest. So they worked with those kids that DID express an interest. Then 3 weeks into the season, the kid said something *before a match* and expected to be able to play that position. With no practice. The coach shut down that complaint pretty quickly and effectively. (Yeah, she called him 'rude' b/c he wouldn't discuss with the kid's older sister before the match about playing the brother at a certain position. Like she thinks that he should stop getting ready for a match to listen to a 12 year old girl about where her brother is playing, when he's trying to get 12 kids ready to play? Seriously?) He also said that if the kids want to play those positions, they need to practice at home. He went on to say that the practice extends to basic fundamentals of the sport, and how he still needed practice with those. He also brought up the swearing aspect and told her that he told all of the kids that they would be benched for an entire game if he hears any more of that. Frankly, I wanted to give the coach a pat on the back for the response that he gave her. And this is a brand new coach who stepped into the role at the last minute b/c they didn't have anyone else to do it! (I think someone needs to remind her that her kid wouldn't even have a team to play on if it wasn't for this coach.)
I really would love for my husband to just kick her out of the league, but I know it's not my call. I mean, I hear negativity (but thankfully way more positive things overall.) But with her, it's just constant. It has spanned years. It's the least little thing, the least little perceived injustice to her pweshus. (She's good friends with another mom who's son IS a star player....he's truly probably the best player in his age bracket. That kid is just naturally talented, but he's well-mannered, is focused, and practices as much as he can. I've wondered if complainer mom somehow mentally puts her own son in the same class as TalentedKid just b/c they're friends, and expects the same opportunities from the coaches.) She must really have blinders on if she thinks that her kid has anywhere near the skill level of some of these other kids. If she kept it to herself, fine, but she makes all of these accusations and mean comments and scathing letters to coaches who are just trying to do the best they can, and doesn't do ANYTHING to make the situation better, like actually get off her butt and WORK with her kid, or help out our league in a different way. I mean, this is how it is. Your kid is NOT a star player, and you're not even doing anything yourself to help him improve. If you don't like it, then by all means, go somewhere else.