To close out the discussion on Korean stock brokers. None of the Korean stock brokers listed above got back to me. I did in the end open an account with eBest in Korea. Below some thoughts on my biased view on the pros and cons:
# Commissions: are as high as Monex Boom. There is a mark-up on the FX between KRW and USD. I am not sure that Monex Boom has such a mark-up, but I suspect they do. No other charges save for commissions and FX mark-up. Wiring money into Korea is about USD 20 per wire depending on the bank you are using of course.
# Account opening: eBest do their best to make the process smooth, but it's not super easy. You will have to get a few documents certified and there are some forms to fill. It also takes a while, assume two weeks if things go fast.
# Orders: only calling/ email works. Website is accessible but you have to submit order by voice/ email for technical reasons.
# Security: Shares are held in your name with the central depository. ebest is listed in Korea and rated by Nice and Kis with a A2+ rating. There is a Korean version of deposit insurance that covers cash and securities up to about USD 50 k. The central depository has insurance as well, but it also has about USD 4 Trn in assets so if the depository fails, there are likely to be problems so grave in Korea that the insurance won't matter much.
Is it worth it vs. Monex Boom? I sleep a bit better this way, but can understand if someone prefers Monex Boom.
@RetroRanger Email him again, he is usually quite responsive. Maybe just overlooked the email. If you continue to struggle DM me, I can give you his telephone number.