Most, if not all, consulates require you to remain in the country you apply from. I suggest you apply for the visa after you have completed your 1 weeks travel
You show invoices to your customers and receipts of money into your bank account matched against the invoices. It also helps if you show tax returns or that you are registered in the tax system in your home country. Also examples of the work you have done and a minimum of 500k baht (or equivalent) in a standard personal bank account.
Extensions are difficult to obtain and immigration don't like doing them, you will need all the information you provided originally but updated to the time you apply for the extension plus 500k baht in the bank for 1 month. Much easier just to cross a border and return
You won't need an extension. When you arrive you will be stamped in for 90 days and according to your ticket you will be inside Thailand for exactly 90 days
There is no option for a 30 day extension on that visa BUT you can get a 60 day extension for purposes of "visiting Thai family" at your local immigration office as you are married to a Thai