Please OP do not listen to half of them on here. People joke about MN users and the majority are level headed people but on threads like this some are trolls and vipers who just want to stir shit or take your words and twist them.
Let's look at the facts you have presented.
You think she is your husband's type. You think. Your husband has not said this from what you have said but you THINK this. Have you had anything to suggest your husband is her type? She has a major long term BF. I think it is safe to say he is her type.
You consider yourself as pretty if not prettier than her. So she isn't going to be winning in that sense.
Your husband has helped her on a few occassions over the course of a 9 year friendship. That is pretty average for friends. He is not leaving you to go see her or do things at the drop of a hat either by the sounds of it. You are worried she has that power over him but has this actually happened. I think her mum dying can be classed as an exception and he came back to ask you and initially brought her to your house first. This all suggests he views you as someone who is on friendly terms with her too.
He stayed at her house over night but only to get some quick shut eye in the spare room until the early hours to drive her to the airport when her partner was unable to do so, you live close by and her mum died. I believe nearly everyone on here has said this was an occassion not to worry. You said he text her twice to check in. I would say this is normal, he wasn't phoning her everyday etc.
She gives gifts to you and your children. You have never told her to stop but sent nice messages saying thank you which suggests to her you ate happy enough with the situation. They are gifts that shows she listens to you as they are things you have said you like. As other Americans on her have said that is how they can behave. You have never told her they are too much, she is not a mind reader. You also disliked that she gave your childten sweets and said your husband was talking about your children behind your back. I'm sorry your husband was probably just talking about his children to his friend. The behind your back is a bit of a tell tale line that you like to control and know everything that is going on and this is one thing you cannot..ironically were you actually on closer terms you probably would but you cannot force grown women to be friends.
She wants to be your friend, you are put off that you and your husband are not both friends with her but then rebuff her attempts to do so, why? I think.you are over analysing her niceness to suit your own agenda and twisting it.
You seem to use her generosity when it suits you. You took up her offer for.your MIL to stay at her house and then were annoyed when your MIL expressed her gratitude. It seems like you are quite controlling. When she went to visit she wasn't just visiting to see your MIL she was on holiday in the area. Depending on how far away you live from your in laws if I was in the area and my friend asked me to drop something off 10 miles would not seem that much of a hassle.
You are annoyed she met a friend of his you have not. Circumstances allowed this to happen, it isn't like he has purposefully not introducing you to this friend. Imagine you found out about this because your husband told you. In all likelihood he introduced them knowing they had that state in common. You do seem to get upset and annoyed about silly things.
With regard to school trips. High schools often run the same trips every year. If there are a large number going then you often have two or more staff organising it and they will run it year in year out. I don't know what subjects they teach but in my school we are encouraged to have staff from a range of departments and faculty involved as all our trips have been interdisciplinary so that the majority of pupils are eligible to go on it which increases uptake and which in turn pushes down the price per person of the trip. It is not some massive scam to spend time together. Knowing school trips your DH and his friend probably share rooms with other colleagues on the trip with them who are of the same sex. Trust me when I say absolutely nothing will be happening on a school trip.
I think the sharing of her letter was out of line. You may not communicate the way she does but she is clearly hurting and to share what was a personal.note to you and DH on a public forum is mean and immature. That she came round with her BF and wrote to and had a gift for you both shows she views you as a couple and she and her partner are hugely appreciative of the support she got that day. She invites you as a couple to her house and it seems all the times your DH and her have socialised out of work it has been with you there too I fail to see why you think she is manipulating anything.
She appears to have been consistently nice all the time you have known her. She is not trying to get your husband to meet her alone or constantly come to see her. There is nothing to worry about.
You are only presenting your choice of one sided facts about her which I feel.is purposefully giving us a skewed picture so we side with you.
You are blowing this out of proportion IMO.