Hi travel-all-the-time . Welcome to the forum, and sorry to hear about your problems

So, to pick up on one point - China does have a higher risk of mileage/points fraud from what I observe (I live in HKG, China). I am quite surprised that Marriott has been so transparent about what the flags were even that caused the transfer, they normally are not so explicit which is probably a sign that they value your relationship. Posting that in public may not be helpful of course to Marriott trying to protect members - There was a post on the opposite end of the scale, where someone was hacked, just a few hours before, which is a reminder of why Marriott tries to do this. I can't however work out why they would be blocking the recipient's account still unless potentially there was something else "wrong" with that account -> That person may have had similar types of transactions in the past that caused other flags not mentioned (I'm a bit unsure from your point 3 whether it was the donor or recipient with stays in China). In case you aren't aware there are people called "mileage brokers" that exist and cause some of these types of concerns from Marriott, and this is quite rife in China when combined with elite benefits and people who aren't the member staying in a hotel but getting status benefits. This is where someone with 400 nights of stays may still be flagged as a concern.

In hindsight the easiest way to have done this particular type of activity (For anyone looking in the future) would have been to gift the night's stay (example here: Award Booking for Others), which would have avoided the points transfer since it was for a single night. I realise there probably didn't seem anything wrong with what you did either btw, just seems to be that is what Marriott is implying would have worked for you.

Hopefully you manage to get this resolved a bit more smoothly as time goes on.