I have come across many companies in Thailand offering foreigners professional services to register a company with all the accounting and bookkeeping support bundled in. A few of them provide legal services as well, though they are not attorneys at law in the strict sense of the term.

Or are these companies "business consultancy" pretending to be accounting or legal firms ?

You don't have to be a CPA to do accounting work; you do to use the designation "CPA." You would likely still have to be an irs certified tax preparer to sign tax returns as a preparer.....99% of the people with Jackson-Hewitt or HR Block are not CPAs. When I bought a house here, I had a real Thai lawyer, but it was actually a lady from the office doing most of the work. Being represented in court would likely be a different matter.