I think it's great that you are helping my fellow US expats. With that in mind I'll try to offer some very picky feedback:
"The US requires that all citizens and green card holders submit a US tax return even if they are living abroad."
This is not exactly true. The US has the same filing requirements for US citizens who live inside and outside the US but there are many situations where you are not required to file taxes. You can play with the IRS online tool for determining if you need to file (
https://www.irs.gov/help/ita/do-i-need- ... tax-return) but as an example, if you are a (nonmarried) single filer and earn below the standard deduction of $13,850 with no other financial life (no large foreign accounts, no investment income) you are below the filing threshold. However, if you are married-filing-separately (common for US expats who marry a foreign spouse and live overseas) the filing threshold is just $5. This is an important distinction because in the US there is no age limit at which the requirement to file taxes starts. If your 3 month old baby makes good money as a model in a tv commercial, they have a filing requirement and that earned money can't go on their parents' tax return. It is only because of these minimal income thresholds that we US citizens don't have to file separate tax returns for our kids as soon as they start making money at their minimum wage jobs (until they reach that standard deduction amount, assuming no investment income).
"One major issue is that most Japanese brokers are not allowed by the US government to sell US listed shares or ETFs to American citizens. They can sell them to anyone else, just not Americans."
I could be wrong about this one but I think the actual rule is that Japanese brokers are required to file US tax documents (a 1099) if they have US citizen customers who own US listed ETFs. The brokers don't want the extra work so most refuse to allow US citizens to hold them. I know that Nomura allows US citizen customers to own US stocks because I own some AMZN in my Nomura account (I chose AMZN specifically because it doesn't have dividends so no 1099 would be necessary) but I believe Nomura would also allow me to buy dividend paying US stocks. (I just did the first steps on Nomura to buy VOO and it didn't block me from buying that ETF but I did not enter the final trading screen and don't know if they would have stopped the transaction.)
