Reasonable means that whatever decision was made is appropriate given the particular circumstances, and most would regard it as so.
Justified means that whatever decision was made is done for a good reason, in the interest of fairness.
There are times the words could be used interchangeably, and contexts where the difference is subtle. It all depends on the nature of the request or issue at hand.
I might use "justified" if I was angry about something, and was acting in response:
Was it polite to skip their wedding after they insulted me like that? No, but I think my decision was justified.
I might use "reasonable" if I think everyone would agree that there was nothing else that could be done:
Was I happy about missing their wedding so that I could go to my mother's funeral? No, but I think my decision was reasonable.
But there are contexts where either word will work just fine:
You said that you'd deliver the cake before noon, but it didn't get here until 3 o'clock. I think my request for a refund is reasonable/justified.
In that sentence, reasonable would work because most people would agree that a refund would be appropriate given the circumstances, and justified would work because the request is fair.
"Here you are" is literal - "This is the thing I'm offering you". It is neutral as to formality. It is brusque rather than polite, but in an informal context, that might just be seen as friendly - it depends on the tone of voice.
"Here you go" is a colloquial idiom, which doesn't really make literal sense. It is informal and friendly.
Best Answer
Here you are / There you are: is commonly used when you're looking for someone (or something) and you found them. For example: you're looking for your brother in the house, and when you found him, you said: there you are!
I've never heard of here are you, but I don't think that's a phrase.
You are here is used when you're basically telling someone that they are... here. You can pretty much replace the word here with any other places.
Here you go is pretty much used when you're giving someone something. Do note that here you are and here you go are interchangeable, but the common usage of here you are is in my first example.