No it is for everyone. Even Visa Exempt ie tourists staying at a hotel etc. Times when they will ask for it is applying for an extension, your 90 days report or applying for a Certificate of Residence. There may be others but I am not aware of.
Of course we had the correct paperwork. It was a multi-billion USD BOI company who provided the paperwork we flew to Singapore with to get the B Visa. It was then extended to 2 years at Chimichurri along with a 2 year WP. This company has been providing the paperwork for 50 years. I think they know a tad more than you do about the required paperwork as they apply for a number each year. Have you ever done it yourself ie applied for a B and had a Thai WP?
It would seem that way. The Ambassador or Head of Consular might be getting twitchy about some of the approved applications and are tightening up. :-) :-)
Not always allowed to convert inside Thailand. Often need to leave to a neighbouring countries. Some countries must even return to home country to apply. I worked for a very large global company who has been BOI for 50 years of em0loying foreigners. Seen it a few times.
If you are Thai Tax Resident it is only monies remitted to Thailand that are assessable. It would need a change of law for it to be taxed on all global income.
You need to talk to a tax expert with experience of both systems but what you stated regarding tax is correct. You would get tax credits in Thailand for any taxes paid in Germany under the double taxation agreement and remitted to Thailand.