- the gathering at sinai did include all jews at the time, according to your source, it just had the addition of the souls of jews for all generations to come.
- in answer to your question, the messiah is supposed to either come in a age of enlightenment, or bring it, as the verse states חֲבַקּוּק ב יד (Habakkuk 2:14) .
For the earth will be filled with the knowledge of the glory of the Lord
as the waters cover the sea.
therefore everyone would be willing to move.
and thirdly on the count of ability, there is also going to be a revival of the dead...
btw even though its discussed in the rambam it originates in the torah Deuteronomy 30: 3-5
3 then the Lord your God will restore your fortunes and have compassion on you and gather you again from all the nations where he scattered you.
4 Even if you have been banished to the most distant land under the heavens, from there the Lord your God will gather you and bring you back.
5 He will bring you to the land that belonged to your ancestors, and you will take possession of it. He will make you more prosperous and numerous than your ancestors.