Apply for visa in USA and then you apply for extension to stay based on retirement after you arrive in Thailand There should be be plenty of information if you search retirement in this group
Simple answer is NO If they ask for proof of location or want an interview you will be stuffed so bet to wait til approved. Most places seem to do it pretty quickly
All the information is available on the Thai visa website but I am surprised that the Embassy in London were not able to tell you. Also the change back to 30 days has not happened and now looks as if it may not happen at all so no need to factor that in.