Each entry is eligible for 1 extension. That said nobody knows what the extension process might entail, such as resubmitting updated application documents. At this point it's advised to exit and re-enter as this process has been shown to be straightforward and smooth.
Why would you apply in Germany if you aren't there? You must physically be in the country whose embassy you are applying to. No need to apply only to the embassy you are a national of. Go to one of the nearby countries from Thailand and apply from there.
Main DTV holder first, then the spouse/dependants reference the main holders visa number. You might email them and ask if they have a streamlined group process for this type of application.
what visa are you on? There is only 1 visa LTR, where 90 day reports aren't required, if you stay in Thailand 90 consecutive days you report, that's the law. Teach it to your tax lawyer. If not then you are subject to a 2000 baht fine
Are you Singaporean? If so you can't apply at this time as a Singaporean national. Not many reports out of Singapore. Suggest you visit the embassy website to find out the visa application process there.