For the Cambodian on the covid extension, provided the immigration office is willing to convert in country and his prospective employer can meet the additional requirements of applying in country then yes it can be converted to a non-B. However, I am not sure if there are additional rules for migrant workers, of which Cambodians are considered.
For the Iranian on a volunteer visa, was this obtained by actually volunteering or by an agency giving a big brown envelope to an immigration officer in the middle of nowhere?
first time I've come across an employer acting like that, my employer did all the leg work for my wife and she just had to turn up to immigration to have her photo taken.
If you haven't already then best place to ask is in the Thai Visa Advice group, although the mods are also in here so I'm sure they'll help if they haven't done so already :)
If you do ask on there then, just to be clear on what you want, ask for how to go about getting your extension of stay as a trailing spouse and that you're currently still on the 3 month visa itself.
Speak to your husband's employer, they should be able to tell you what you need to do.
Edit - are you currently on an extension of stay or only recently arrived to Thailand? Looking back at your post I'm actually confused as to what you're trying to do.