Mauna Loa erupting

[My 2 cents]

It's really too bad that a barrier couldn't be built to prevent this flow from oozing across a major highway.

That barrier would have to be big, long, tall. The topography along that stretch of the saddle is pretty flat, and the lava itself viscous enough that it isn't just running around. Instead, as it encounters barriers it's going to pile up and climb over them too.

I'm imagining a large perched pond, a 'secondary' lava lake, forming before the flows get over to the Mauna Kea side. In that case where the lava goes from there can be in any direction. Whichever side of the levies that form is breached first.
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Seems like encouraging the breach to happen at a location that would cause the least amount of damage would be the better way.

That will never happen.  So it will start a new guessing game of where will the breach happen naturally?  Most likely (my guess) is that it will happen towards the opposite end of the flat area from the flow front.  Which side is anybody's guess, with drastically different results.