I have just three questions to consider for this thread.
1/ To what extent are beliefs about the Second Coming of Jesus beneficial or harmful?
Interesting topic.
It really depends how people are looking at it. I think the negative aspects of it are if people take it literally, as though some outside agency is going to come and act and take care of business, as it were, it keeps them from being a participant in the system in anyway that has meaningful impact to change the present situation. Rather than the Kingdom of God being an imminent, inward unfolding into an outwards world in which they participate in bringing that forth, it is viewed as being enacted by some external force. It removes the inward and upward thrust of spiritual life, of which they need to actively participate, and denies it to ourselves by imaging God's going to "do it" because we can't.
So the negative there is to not realize the active nature of spiritual life, forestalling it for "kingdom come", another time and place, beyond us. We don't grow spiritually, and the kingdom doesn't come.
On the positive side, if someone imagines the second coming, as a continuous coming into being though Spirit, "thy Kingdom come, thy will be done, on earth, as in heaven," places that right here on earth in the now. So it becomes a realization of a spiritual truth, rather than some future eschatological perfection to come. The perfect is coming, as we do "thy will on earth as in heaven". It is a metaphor for a spiritual truth. That leads one as a truth to a positive position to actually do good in the world. That to me is what being a Christian should be, rather than waiting for some future deliverer.
And with that voice, I would call myself speaking as a Christian, even though I don't care to identify myself with any one religion. It am speaking as I hear the ground Christian message, beyond all the theologies and dualistic beliefs.
2/ If the concept is to be taken seriously, what can we reliably determine will be the distinguishing features of Christ when or if He has come?
One can take the concept seriously, without needing to take it literally.
How can we determine if Christ is legitimately being "returned"? By the authenticity and the fruits that they bear. Those will speak the truth of that person, and anyone else who claims to be Christ. I'm of the view that one can be Christ, at any time in our lives we are fully surrendered to God. "Thy will, not mine", when enacted we become Christ.
If someone as an individual claims to be THE returning messiah to the exclusion of all others? Then I'd say they think rather highly of themselves.
3/ Are there lessons to be learned from history about the acceptance or rejection of previous Messianic type figures including Christ Himself?
Yes, history does teach of the folly of looking here and there for the Kingdom of God when it is here among you, said Jesus to those looking for signs.