1] understand what this leads to:
These thoughts give birth to endeavors as these thoughts are the starting point of action.

Whoever guards or looks over these thoughts he will take the yoke from himself and be freed from his desires. If a person's thoughts overcome him then his desires will be over him. Likewise, whoever is easy going with these thoughts, they will forcibly lead him to his distraction. These thoughts will go back and forth in the heart until it becomes a false desire to do something about it.

2] understand that it develops into a false desire:

These imaginations will develop into something of a delusion, to the point that the person will believe it to be real.

Ibn al Qayyim said in Jawab alKafi that the most despicable people in commitment to desires, and the lowliest of them in spoiling the heart of being subservient to desires is the one who is pleased with the false hopes that these thoughts may bring about over the realities.

Taking these false hopes and adorning the self with it is the capital investment of bankrupt people. For those who trade in falsehood, these thoughts are their main investment. These thoughts are likewise the sustenance of the empty soul that is totally content with living in an imaginary world, away from the realities.

3] Indulgence in these thoughts give birth to laziness, shorthandedness in ones affairs, and extreme regret for the things that have passed him. Out of this, he will now choose to live in this fantasy world. This is like a person who is hungry and imagines he is eating food and drinking water while he is not.

So the purity, highness and cleanliness of the soul comes by flushing every thought that has no reality to it. and not being pleased with anything that comes to his mind will lead to this.

4] Understand the four kinds of beneficial thoughts:

a. thoughts that causes slaves to obtain the worldly needs to live
b. thoughts that obtain benefits in the next life
c. thoughts that will repel the things that will harm him in this life
d. thoughts that will repel the things that will harm him in the next life

So the slave of Allah should restrict his thoughts, his ideas and aspirations to these four categories only. If he restricts them to these and does not accept other kinds of thoughts, and if he emphasizes the most important of these thoughts, then he will obtain the reality of the jannah.

5] Understanding the importance of Intellect over Matter:

The slave of Allah should only use his intellect and his ideas over his desires, making certain that these do not leave reality, the legislation of Allah, and obtaining the benefits of this life and next. Thoughts for Allah alone and next life is of different types:

a. Thinking about Allah's ayat that have been sent down, understand Allah's intention behind them, and acting upon those ayats.
b. Thinking about Allah's ayat, his natural signs, and reflecting upon them. We use these reflections as evidence for his mercy, His names and attributes. Allah has encouaraged us to contemplate and understand his signs, and condemned one who is heedless of that.
c. Thinking about His bounties.

These three types of intellectual pursuits bring about the love of Allah and the knowledge of him. Constantly thinking about Allah in these ways in combination with dhikr of Allah brings the heart both knowledge of Allah and love of Allah.

d. Thinking about bad qualities of the self and his deficiencies of actions. This type of thinking has great benefit! Ibn al Qayyim says that this type of thought is the door to all good because it has a great effect in breaking a part of the soul that commands evil. Once this part of the soul is broken, the soul will live in content and light will be brought to the heart giving it life. The heart in turn emanates this life to the rest of the limbs.
e. Thinking about the obligation of that particular time. If the slave of Allah wastes that time, all his benefits will be wasted for all the benefits only come forth from time. We know that when time is wasted it will never be brought back again. There is a saying: "If u don't busy yourself with the truth. It's going to busy itself with falsehood."

So the time of the person is his life and reality, by how he spends it is how he will leave it. This time passes faster than the clouds so whoever spent the time for Allah, then it will be his life and whatever is besides that is not to be considered from his life even if he lives a long life since he lived the life of animals.

6] Understand that in the end the slave of Allah will only get what you put for Allah, thoughts are either for Allah or not, either truth or falsehood delusions.

If the believer spends his time in heedlessness, desires and in false hopes, and the best thing that he ever did was just sleeping and staying away from haram, he might as well be dead because being alive just decreases the person instead of increases him.

7] Understand that heedless thoughts will harm only if the person indulges in them. Meaning, if these thoughts are invited in the head, and being thought about in length then they become harmful.

So the passing thought is like the one who passes by a road. If you do not invite him in, it will pass by you. And if u were to invite it in, it would spell bind you with its speech and its delusions.

This is the lightest thing upon that empty soul that welcome all thoughts enter it, while it is the heaviest upon the noble soul that it enters upon.

8] Understand that these thoughts are enhanced if not originated from unrestrained glances of other people.

Ibn al Qayyim says that the unrestrained glance is the basis for all the general bad occurrence that affilict human beings because from the glance comes the thought, then from that thought comes the idea, and that idea gives birth to the desire, and from that desire of the heart comes the intention, and that intention will be nourished by these thoughts until it becomes a decision to take action and the person commits a sin.

9] Contemplate about death before going to sleep for the prophet (sallallahu alayhi wa sallam) described death as the destroyer of pleasure. Hence instead of thinking about two people having intercourse, think she should imagine being buried and her loved ones are leaving her behind under the soil. InshaAllah this brings many benefits one of which is that she reflects upon her last deeds for the day and what she can do the following day to rectify her situation.

10] After gaining these various matters, she should engage in various beneficial activities such as constantly carrying a mus-haf with her to read most especially at night, she should make plenty of dhikr and supplications to Allah. Perhaps volunteer to help an elderly woman at home on certain days or other strenuous permissible activities so that by the time it is time to sleep, she will neither have the energy nor the time to engage in these thoughts.

Allah knows best.

Question 2:
Ihtilam (seeing an erotic dream) is not haram in part because it is natural.

It was narrated from Zaynab the daughter of Umm Salamah that Umm Salamah said: "Umm Sulaym came to the Messenger of Allaah (peace and blessings of Allaah be upon him) and said, 'O Messenger of Allaah, Allaah is not too shy to tell us the truth. Does a woman have to do ghusl if she has a wet dream?' The Prophet (peace and blessings of Allaah be upon him) said: 'Yes, if she sees water (a discharge).' Umm Salamah covered her face and said, 'O Messenger of Allaah, can a woman have an erotic dream?' He said, 'Yes, may your hands be rubbed with dust. How else would her child resemble her?'" (Bukhari)

Is it haram because even if she purposely developed the habit?
Bad thoughts are not haram in and of itself as long as one does not act upon them. Allah (azza wajal) revealed:

"Allah burdens not a person beyond his scope. He gets reward for that (good) which he has earned, and he is punished for that (evil) which he has earned. "Our Lord! Punish us not if we forget or fall into error, our Lord! Lay not on us a burden like that which You did lay on those before us (Jews and Christians); our Lord! Put not on us a burden greater than we have strength to bear. Pardon us and grant us Forgiveness. Have mercy on us. You are our Maula (Patron, Suppor-ter and Protector, etc.) and give us victory over the disbelieving people." 2:286

Thoughts in Islam are not going to be against you unless you act upon them or intend to act upon them if you had the capacity and means. But for the reasons stated in Question #1, it is good to keep away from "purposely" having these thoughts for it can lead one to bring them into action especially in times when the Iman is low.