Roth conversion as an NRA might either be useless or actively harmful, you need to figure out how all these accounts are taxed *in your specific situation (i.e. country) first*.
The key thing to note is that US taxes aren't the main issue, taxes in your country of residence are generally the sticky point.
The first thing to do is check whether your country and the USA have a dual-taxation agreement. If there is one, it will govern how withdrawals from 401k's/IRA's/Roth's are taxed. If the US doesn't tax the withdrawal you can essentially ignore US tax law, in other cases things might get more complicated. (RMD's I'm less sure about, but since you aren't required to file US taxes it would be hard for them to even apply the RMD.)
As it turns out, Canada does have a dual tax treaty, and you can find out a little more about Roth's if you search the web. Here's the first link I found:
https://www.expattaxprofessionals.com/roth-ira-taxation-expats-canada/Now that link doesn't talk about Roth conversion, but based on the next link it sounds like only Canada taxes any withdrawal (modulo a US tax withholding which you claim back in Canada). Since only Canada is taxing the withdrawal, you only need to care about Canadian tax law, and it sounds like a conversion won't really work.
http://madanca.com/blog/steps-withdrawing-ira-401k-returning-canadian/TL;DR: RMD's might not even apply, reducing the incentive to convert to Roth - but you can move the money out of the 401k/IRA and into Canada to completely avoid worrying about the US.
For "Southeast Asia" I can't really help, but similar to the example above: you just need to figure out tax agreements between country of residence and the US, and taxation in country of residence, and you'll be much better prepared to figure out taxation. As long as someone holds a greencard they'll be taxed in the US though, regardless of any additional taxes that the US assesses (in some cases, the country of residence taxes first, and that tax is credited towards the tax the US assesses on retirement withdrawals - but it's a country specific thing).