Living in Thailand using the Extension of Stay based on unable to travel and/or the 90 days bilateral agreement you get on entry does not sound like the intention of the making of the rules.
To avoid all questions and fears of getting into a country is using the visa that fits the purpose of your stay.
There is no way we can tell if you can enter without any question, with some questions which some people call a real hassle. It is the job of the Immigration Officer to check people willing to enter the country.
There is no detailed description of all the methods to do banking.
It must be in an account where the money is accessible to withdraw in case of emergency. Talk to your local Immigration Office if they have any problem with an account in sterling.
First you apply for conversion to a marriage visa and after 60 days you apply for a marriage visa, so it is from day one possible to apply for a Work Permit with a stay based on marriage visa.
In official wording: First you apply for a Non Immigrant O visa based on being married with a Thai National and after 60 days you apply for the 1 year Extension of Stay based on being married with a Thai National, both called marriage visa as this saves time in typing.
If you quit your job you have to cancel the work permit at the Ministry of Labor and your Extension of Stay based on employment at the Immigration Office.
If you are legit to apply for a new Extension of Stay based on another reason depends if in this case the school can provide all the requirements in time and on your initial visa. Check this with the school and Immigration.
By Thai law the owner, landlord, house master need to register foreigners residing at the address within 24 hours. We do not know if your extended family members are Thai or foreigners, but as you mentioned you do not want to do anything wrong, download the app, register the address and each time a foreigner stays there you an report it within 5 minutes using this app. it is easy, quickly and according the law so nothing wrong.