Typical Thailand. Its never straight forward. Local immigrations often have their own rules. Asking is for sure the best thing to do. In every situation conserning the immigration.
Tourist on visa exemption or tourist visa not need todo a 90 days report. If the immigration office did not inform you to make a 90 day report then you not need to do it. You are already at the immigration at every shortterm extension. But it can not harm to ask.
Your first extension of stay is also your startingdate for the 90 day report. This is only the first time you get the extension. After that its a countinious 90 day cycle or starting new whenever comming back to Thailand from abroad.
I hope you can get a new passport within a month. It seems to be difficult nowadays. UK governement recently talked about 10 weeks. Start the proces asap and see if there is a way to speedup/fasttrack this proces.
By air there are no limitations in the number of visa exemption entries. That is, till immigration assesses the amout of back to back visa exemptions are to much. At the discretion of the immigration officer.
Booking schedule is not important. You can change as much as you want. As long as you stay within the limits of your permitted period of stay. Stamped in your passport.
The main point is that the Netherlands is one of a few countries who do give a verrified legal income statement to who provide evidence of income to the Netherlands embassy in Bangkok.
If he is from the Netherlands and he can show 40K (retirement) income to the Netherlands embassy in Bangkok. Then they will supply a income statement valid to apply for the Non O and 1 year extension.
Het heet : visum ondersteuningsbrief" aan te vragen bij de ambassade in Bangkok.