This is a very interesting topic. I hope others will join and contribute.
I will make a few points:
1.
I think that whether republic or monarchy is better is depends on the changing character of the society.
So it would be inappropriate and harmful to try to introduce monarchy in Rome of 2nd century BC or in XIX century USA. And it would not be a good idea to try to introduce republic in the same Rome of 3rd/4th century after Christ (at the time of Diocletian) or in XV century England.
For each system there is good time and place.
2.
"Going back" to the previous system is usually impossible. BUT IT IS POSSIBLE to get it again in a NEW (often disguised) form by going toward future.
Roman Republic started same way as United State - by revolution against the monarchy. Romans were very passionate about their republic and did not take kindly any talk about return of the king.
But after several centuries they got monarchy through the BACK DOOR. When their republic become powerful and dominated other Mediterranean societies it became necessary to develop monarchical practices in order to manage the Empire (what was the ancient name for the International Community or system of alliances with Rome being the hegemon).
3.
Still it was not perceived by the majority of Romans - long time after Caesar they believed that they are free citizens and that they have the republican system.
Some of the first emperors tried to restore the republic but they could not go against the needs of the time. They could not overcome the natural law that for each society at specific time there is a natural system or state of equilibrium.
Of course, when the given system is firmly established the general public is convinced that it it the best and universal system for everybody and for ever. People usually cannot imagine different world than the one in which they live and they take appearances for reality.
4.
It is likely that the only way to preserve the American republic is to reduce foreign involvements. I think that Buchanan is very aware of this dynamic and he is a true republican. But if the history is to be repeated he will fail like Cato and Brutus did.
See my tagline.
When you speak of "The Needs of the Time", I would say that, in truth, there is nothing about republicanism per se that makes it unsuited for times when monarchy has held sway, or vice versa. Rather, government's success or failure is determined almost entirely by the personal qualities of the leaders who run it.