I would have contacted the embassy and asked them if they would allow a trailing spouse visa on a Non O-A. Not all embassies still issues this. But your plan in general is correct and here are the requirements for a 90 days Non O visa visiting non Thai family also called trailing spouse visa.
Onward ticket has always been a formal requirement for tourist entries, but mainly a airline issue and rarely checked by the immigration. Now when things have become more strict and they might look for a reason to deny you, they could ask for an onward ticket and use that as an official reason for denying. Best you can do now is to make sure you fulfill formal requirements and have an onward ticket, proof of some nights accommodation and funds for your stay.
The rules regarding tourist extension are anyway quite clear so no reason for 1000 different answers. Some immigrations as Bangkok and Chiang Mai allow you to apply for an extension 45 days before your current stay expires. The rest allows extension within the last 30 days, except Jomtien and Samui who accept extension within the last week.๐