"Steve Bannon wrote that three people: Palin, Sessions and King would decide the conservative choice. "
A very reasonable scenario. Ted Cruz is ahead or tied with Donald Trump in Iowa as the caucus day nears. Sarah Palin flies to Iowa, attends a Cruz meeting and announces that she is wholeheartedly endorsing him.
What would anyone, with any political sense, imagine the ensuing result would be?
It would be the same as happened for candidates Cruz (for senate)/Sasse/Ayotte/Haley/Fischer et al, there would be a flood of money, feet on the ground support and a rise, probably substantially in the polls and victory.
Iowa is of course a state where evangelicals play a major role in the selection of the winning candidate, see Santorum Rick, and if there is one person who has the ear, if not wholehearted support of evangelicals it is Sarah Palin who constantly walks the evangelical walk.
Now if Donald Trump was strongly challenged in this scenario, having a shift of support by evangelicals from Cruz (and Carson's residual support), who does well with them, to Trump would give him the widest body of support across groups and the needed votes. Palin could be exactly what Trump would need to effect this:
"Tea Party respondents supported Trump at slightly lower levels"
In a close race a shift, of even a small number of evangelicals to Trump, could make the difference. Again, if Palin flew in and endorsed Trump in Iowa I would be utterly confident that would be enough to give him the win.
Palin's influence would not be anywhere as strong in New Hampshire, but on present polling with Trump in a substantial lead she would not be needed there, in fact it might be a negative in such a liberal state.
However with South Carolina up next the exact same scenario as in Iowa would apply. Then, on to Florida where Palin drew massive crowds in 2008 and, in my opinion, kept Florida relatively close for the doddering McCain campaign as the red parts of the state turned out comparatively better than other states.
If this scenario played out, then at that point the race would be pretty much over. The lesson for Trump would be that in these key states Palin would be an essential ally.
Whether that would be as his running mate, or with a major convention role and the confirmed promise of a key cabinet position, would be not only up for consideration but would be, in my opinion, and under these circumstances, up for a positive conclusion.
Trump is far to smart to repeat the ridiculous decision of Mitt Romney and his team to exclude Palin from the 2012 convention and the campaign.
A Palin endorsement and active support commencing in Iowa could be instrumental in getting the 2012 stay at homes to the polls and give him the victory in November 2016
