the embassy website will have a list of the companies which are accepted by Thai customs. AXA issues a bare bones policy that meets the requirement. That’s the cheapest option
Provide the HK police with a printout of the requirements from the Thai consulate website. You have to submit the police clearance certificate. The HK police can’t submit it directly to any Thai government agency as they would just ignore it
The US doesn’t do withholding on dividends. Of US dividends taxes are paid when you file. That 15% is the German authorities withholding. As with all things german it may take 2 decades, your first born, 3,000 paper reams and the alignment of all the stars to ever get that 15% back. It can be done but just try googling around to the German government websites involved to get an idea of just how many years of your life it will steal
If he thinks getting married is the best thing to do then that’s a terrible option. Even the. He’ll have to go thru all the visa rubbish. Best to get any of the other visa options.