If I were a Republican, I wouldn't even want Ernst to win. She'll harm the party by being such an embarrassment way more than her individual vote will help.
One senator either way, even when "control" is at stake, means very little. The exact count only matters when one party is approaching 60, and that's pretty unlikely to happen either way in the next 6 years.
And yes, I would say the same thing in real life if the Democrats had nominated a liberal equivalent of her in one of the close races.
So youre suggesting the people of Iowa write in Mickey Mouse?
Braley isn't a conspiracy loon, he's just an incompetent campaigner. I don't anticipate him embarrassing the party once he's a senator (although to be fair, you can't always predict these things).
But yes, if the Iowa Dems had nominated a Dennis Kucinich-type, then yes, I would endorse a third-party candidate, and had, say, Kentucky nominated a Kucinich-type, I would endorse McConnell.
The only time I would deviate from this is if one party were on the verge of 60 seats.