The business registration issue is worth asking someone knowledgeable about. We had a brief discussion here:
It’s true that the government is very unlikely to investigate alleged sham contracting if no-one files a complaint. Still, if you are an employee your rights and obligations for income tax, labor insurance, health insurance, and employment insurance are different from those of a contractor. Even if you never have health issues etc., you will need to file tax returns.
Registering your own business will count in your favor if you ever have to defend your status as a contractor. If you ask the Ministry of Labor for an explanation of employment vs. contracting, they will basically say an employee is a subordinate*, and a contractor is not. It’s easier to argue that a business-to-business relationship has only a minimal degree of subordination. If you’re interested, see the Civil Code: employment is “hire of services”, not “hire of work”. There’s also “mandate”, “publication”, etc.
*personal subordination, economic subordination, and group subordination; also, the requirement to perform work personally instead of farming it out to a third party is often cited