Ok. Apply on the 15th of March. Probably a good idea to go before and ask for the hand out of what’s needed so you can tick all the boxes at application time.
then you do indeed go to the main government complex at Chaengwattana. Desk J deals with tourist visa extensions. Hopefully you’ve already booked an appointment online. Otherwise it may be a bit of a wait to get service.
That depends on your immigration office. Some may let it slide if you don’t have one. Some will insist. Tell us what office you use and perhaps some will have experience with that office’s policy.
You can get a dependant visa/extensions based on your husbands visa. Usually they’d need to be on the year extension rather than the initial Non B but that could be immigration office specific.
You’ll need agreement and documentation from his company to allow you to do so.
Check here for requirements to do so in Phuket. Other offices may differ slightly but it’ll give you some idea.