Out of office and working on Friday, respectively. However for the first case some people might use that and then be in the office on Friday also.

I prefer, personally, "It's better to regret something you've done than something you haven't" although I couldn't say that the tense you used is incorrect.

I don't know that it's incorrect but it just sounds weird, bro. That said I don't see how I could argue that fear should be different from other verbs in the past tense like wanted, needed wished, desired, etc. So I would like that an English teacher could say something about this, also.

I would use "go into detail" for this. I'm not actually sure on the rule behind this but it sounds better to me here.

I agree that this can be a confusing case. I'm going to have to agree with Lee, and I'll explain why in a moment here. Yes, due to how the commas have been used, this can look like a mess and confuse us as to which noun is being referenced. However, we'll have to take another look and see that, from the get-go, "hardcore graffiti" is the subject of this excerpt. Additionally, if we look at the words contained in the parentheses, they're being used to describe hardcore graffiti and you can see this by the singular possessive "its" which means it cannot (at least not without being grammatically incorrect) be referencing the plural "taggers" or "writers" or "sprayers". Another reason why this is confusing is because immediately the writer switches to speaking of the possession of the graffiti by the "taggers, writers or sprayers" and we can see this by the use of the word "their" which is third person possessive plural. Yes, we might hear (especially when the gender is unknown and one wants to avoid being offensive by using "its" which is typical with pets) someone refer to something possessed by the subject as theirs and in my case where theirs would be correct. However hardcore graffiti, being impersonal should not be referred to by "their". Furthermore, as we continue in the sentence the writer again immediately switches subjects they're speaking about and again references the graffiti saying "is not" which is referencing, again, the singular. We can also see the writer is not referring to the artists here as artists can not be used to gain respect of fellow artists, but art can. I apologize if this took a while and I hope I've explained thoroughly and cleared up any confusions there might be.

To go off is just a weird way to say to go and do something. He went off to college. She went off on him (this means she "told him off" or she scolded him, etc). He went off to golf (he went to go golf or yes as you said went out to golf except in this case goes off really stresses the leaving and and doesn't necessarily imply a return in the future whereas went out to golf pretty much does). Go down is a similar situation. Down is basically useless it's just an added word. He went down to the IRS office to deal with his stolen identity issue (He went to the IRS office or he went over to the IRS office, etc).