Perhaps a process of elimination might help determine which of the possibilities you suggest cannot hold good, leaving the answer standing clear.
1) Is "Be perfect" a summary of the Sermon on the Mount?
This section starts at chapter 5 verse 1, where Jesus is on a mountain, his disciples approach him, and he starts to teach them. The section ends at chapter 7 verse 29, for the next verse says Jesus came down from the mountain.
It is hardly possible for a summary of three long chapters of teaching to be given in the middle, and that statement is right in the middle, at chapter 5 verse 48.
However, if you mean whether "Be perfect" could be our way of expressing a summary of the whole Sermon on the Mount, that is really a different question. That is because the phrase, "Be perfect" (as Father God is perfect), is only stated once by Jesus in the middle of his discourse. Jesus chooses to end his instruction with the parable of the foolish man building his house on sand seeing his work destroyed; such is the man who hears Jesus' words but does not obey them. That really sums up Jesus' teaching on the mountain - having heard, go do.
2) Is "Be perfect" the conclusion of the teaching on loving one's enemy?
Certainly, perfect love includes loving one's enemies, but there is a lot more to it than that. Jesus said in John's gospel (15:13) that there is no love greater than a man laying down his life for his friends.
Jesus mentions 'love' three times in that bit in the sermon about one's enemies - chapter 5 verses 43, 44 and 46. Then he goes on to warn about hypocrisy, teaches his disciples how to pray, and many other things.
So, there is some warrant in thinking that when he says "Be perfect" at the end of those six verses speaking about one's enemies, that could be his conclusion on teaching about loving one's enemies. The only difficulty with that is that we have made an artificial division here between six verses teaching about one's enemies, and all the other instructions contained in about 20 previous verses. One could equally say 5:48 is a summary of all the verses before, up till then.
3) Is "Be perfect" a stand-alone commandment?
There are some clear 'stand-alone' commandments in the Bible, such as "Do not murder", "Do not steal", "Do not commit adultery", "This is his commandment, that we should believe on the name of his Son Jesus Christ, and love one another" (1 John 3:23).
However, "Be perfect, even as your Father which is in heaven is perfect" cannot stand alone, because it requires all the details Jesus has just expounded to make sense. Even more significantly, it requires the enabling of the indwelling Holy Spirit in believers to understand, and then to strive to be.
Conclusion: Because understanding of that instruction only becomes clear once we have understood what Jesus meant by it, we are putting the cart before the horse trying to choose one of the three options suggested. Only once we have been enlightened as to what Jesus meant by that saying can we know if it requires to be 'positioned' in any respect to our thinking about the Sermon on the Mount. Although it is a clear instruction, it does not apply 'only' to loving one's enemies, as Jesus shows in all of his teaching (not just that sermon, at that time, in that place.)