in relation to pricing.... I have social security and my stepson is a minor so probably best not to expect same price if you don't pay ss. But I've never heard of them ripping off foreigners.
my stepson had one 4 days ago in Takuapha hospital (Phang Nga province) and it cost me 620thb. (he is Thai. I am not aware of dual pricing at this hospital)
just double check on one of the Samui fb pages or the Kpng pages just before to double check just incase there are last minute changes, but it's been like that for severalonths now
if you get your extension in Samui or Koh phangan then you will likely have your 60 day stamp on the same day for 1900thb. Your tm30 would have been submitted by the accommodation where you are staying.
as far as I know, rasibg half Thai child does not prevent you from working. The work permit has nothing to do with immigration. It's a matter for the labour deparent.