Never had issue at immigration. Only had problem with checking in to the flight at airports, in which case I just bought a fully refundable business class ticket to show the airline and had it refunded afterwards.
You need a letter from your country’s embassy to support your visa extension for Thailand. Also there are still flights going into China. Having legitimate work with academic institution in China should still enable you to return. Speak to your employer.
This is the offical list of docs in Bangkok. You may be able to do it if you read Thai (all the companies papers are in Thai only). I believe very few people do it without an agent at immigration.
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