Dream

Well, people dream all sorts of things, some of them have meaning and some of them don't. If she is praying for this and you know she is praying for this, it makes sense that you might dream about it. Possibly your subconscious is thinking about it even if your conscious mind isn't.

If her brother has expressed an interest in marrying you, and it is a reasonable possibility for you to marry him, you could consider it. It is a blessing to have an opportunity to marry someone compatible whom you might have some trust in. It could be nice to marry a friend's brother (although keep in mind it can also change the nature of your friendship, especially if there are marriage problems). Of course there is no guarantee that the marriage would be happy but it is worth considering if you think he might be a compatible spouse. However you should make your decision based on the real life circumstances around you, not just the dreams.

If he hasn't expressed an interest in getting married then there is no reason to give it thought - he may or may not be ready to get married right now.

Anyway, you are lucky to have a friend who likes you so much that she wants you to be family. :)

A lot of things are said about what is and is not possible regarding dreams about Ahl al-Bayt (A), however, as far as I know, most of them have no basis.

It is possible for the Shaytan to appear as something he is not; in fact, the main feature of Shaytan is deception. 

However, even if you were dreaming about something other than Ahl al-Bayt, there are a lot of things between Ahl al-Bayt and Shaytan that you could have been dreaming about. (Including your own subconscious!)

If, in your dream, the being told you to do something evil or which you know is wrong, it is good to ignore it. It doesn't really  matter where it came from. It could have been Shaytan or something else. 

If it told you to do something good, it is good to accept it.  

If it told you something else, you can also look into it and see if it is a beneficial or destructive thing. 

If it just stood there and didn't do anything, then it doesn't really matter - all you can say is, you don't know. 

So, I don't think you should worry too much about Shaytan being in your dreams. While it is good to be conscious of Shaytan's existence, it is not good to get too paranoid about it (indeed, that could also be a trick of Shaytan!). I think Shaytan is probably busy in other places and maybe doesn't have too much time for dreams unless there is some very specific reason why you think this is the case. 

(This is assuming you actually had a dream about this and are not just asking in general)

On dreams:

Imam al-Baqir (A): ‘When people sleep, their spirits go out into the heaven. Whatever the spirit sees while in the heaven is true, and whatever it sees in the air [between earth and the heavens] are just muddled dreams.’

Imam al-Sadiq (A) said, ‘There are three types of dreams: glad tidings from Allah for the believer, ominous dreams from Satan, and muddled dreams.’

So, yes, some dreams have meanings (as you mention you have already experienced!)

Certainly, it is possible you saw Imam Ali (A) in a dream. Generations of people have done so before! It is also possible that you saw something else that was trying to comfort you, and it presented itself as Imam Ali (A) because that is what your mind would understand. It is really hard to know, and maybe it doesn't matter.

Usually, people dream about environments that resemble what they have seen in real life (or sometimes on television). Since this dream was about a strange place, it is possible that - as the hadith says - your soul wandered off somewhere further away from the normal realm of existence and found this frightening or difficult to understand. This happens sometimes and doesn't need to have any particular meaning (for instance, being a prediction of the future), but it does have a reality. Since it sounds like it was a particularly strong experience, perhaps this was the case. 

It could also refer to something that will happen in the future, and perhaps a meaning may unfold in the coming months or even years. It sounds similar to some of what is narrated about what happens when the soul of a believer transitions to the next world, in that the believer will see Imam Ali (A) as a comforting figure, so it could also be a premonition of what that experience will be like. (I am not saying that you will die soon, only that it eventually happens to everyone, and sometimes people have dreams giving them a flash of what the afterlife, heaven, or hell are like.)

In any case, dreaming about Imam Ali (A) is a good thing and the message sounds positive! Dreams can remind us that divine assistance and intercession are ever-present realities and we are not alone in this world, and this can be a reassuring thing. 

I believe that the dreamer best knows the significance of their dream, and, often, by telling your dream to someone, you come to understand it. I am sure that all the meanings you would think of are positive!

However, it is also possible that your soul was wandering a little during sleep and saw the plum tree at a higher level of reality in an ideal or perfected form - that is, what it would look like if it were not subjected to the imperfections and decay of this world. This might also be why you remember it strongly. This is similar to how, in Paradise (inshallah), when we see a person or an animal or a plant, we will see it in its ideal and perfect form without the defects that they have in this world.

It is a nice dream anyway!

Yes (as long as there are no reasons why they are forbidden for marriage, for instance they are mahram).