Whilst I definitely would try to live in harmony with my neighbours, I also think that sometimes people do not use common sense or think into the future. If your neighbour came to live there after you, she would have seen your garden at the time and could have thought what it may look like in the future.
If you have large gardens she has other places where she will get more sun and could actually use the shade area to be able to plant different varieties of plants .
You have done your best to be a reasonable neighbour, but she seems to think she has the right to tell other people what they can and cant do in their own garden, so she may then decide something else she doesnt like and it could be neverending. You have tried to be accommodating but it is your place, so perhaps you could talk to a gardening club or friend who might have good ideas that you may not have considered. I would just look out a couple of photos of your garden from about the time she came to live there, so that if there is any further problems you can use it to point out the situation as it was in the beginning.
I think in general there seems to be a quite selfish attitude now, where people think that once they have moved somewhere then they expect things to change to suit them. You see this especially in the countryside where people long to come and live in a village or out in the dales or wherever and then complain that animals keep them awake, or move slowly on the road or tractors get in their way etc. Of course estate agents are there to extol the virtues of places but sometimes I think there should be somewhere people could go to check up on the possible downsides that they have not yet thought of.!!
I hope you can come to a good resolution and we all need to help our wildlife as we are losing so many these days.