Well, I think forex and bankruptcy of a country might stand on different ground. If a country goes bankrupt, their currency will depreciates so you can go short on it of course. You might not want to deposit more than $100 during those kind of hard time. You just never know if they will close off their company and scam your money or steal from you or just have some laws that will come in and interrupt the business of forex and the brokers. Well, that is less likely to happen in this day and age. However, you do not want to deposit so high in any broker, and remember events like LR, where your money was lost. Also, bitcoin recently had large illegal scandals too so anything can happen so protect yourself first.
Yes in cases of bankruptcy in a country, they might come up with some kind of laws, limitations that will affect the broker and investors as well. The broker might stand clean and keep a clean sheet but because of the laws that will be put in place to safeguard the country's economy at that time, the traders would most likely have some negative part of it.