So many questions. Let's start with the first one:

Do we really need to move billions of people?

We humans couldn't live under current conditions without the tech we built. Why can't we build the tech to cope with new conditions? Put roofs made from solar panels over peoples heads and air conditin below the roofs?

I think it's not so much about us adapting to the new climate, obviously we can figure that part out. It seems to be more about our systems adapting fast enough to keep up with the changing climate. How many crops this year were plagued by drought? Were we able to produce enough food? What about livestock, how did they fare? What if those droughts continue to get more frequent and longer?

You forgot to mention how you build the rest of the boat, the part that goes under the solar panel roof.

> a quarter of Bangladesh’s landmass, bound on the south by the northern Indian Ocean, hovers less than seven feet above sea level. And as floods grow increasingly frequent and severe, the 163 million who make their home in Bangladesh, the world’s most populous delta, know little escape from water.

https://www.nrdc.org/onearth/bangladesh-country-underwater-c...