Caveat: I am not a lawyer and this is not legal advice.
Tl;dr: Drop the "legal", use only Nachname and Vorname.
Legality of using a false name
According to Section 111 of the Ordnungswidrigkeitengesetz (DE), when you give a false name to the authorities in response to an official request it is a fineable offense (fine up to 1000 euros). Giving a false name to anyone other than the authorities is not a fineable offense. However, depending on the intent (i.e., illegal enrichment) it may expose the person to criminal penalties (Section 263 of the German Criminal Code (EN)).
False name does not invalidate contract
I am not aware of any statute or case law that would invalidate a contract entered into under a false name. Order a big-screen TV set under an assumed name and you are still on the hook for the sale price.
It can be taken as given that when you are asked for forename / surname, it is the real name that is being requested. (A special case is if you are, e.g., an artist well-known under a different name, which may even be included on your officially issued ID.) As evidence for this contention I offer the following particulars:
The case of Heise Verlag
In 2006, German publisher Heise Verlag prevailed in a civil proceeding (DE) in Munich Regional Court. The court recognized the right of Heise -- the plaintiff in the proceeding -- to terminate the membership of a user of their online fora. The individual had registered under a false name. The court held that although the parties had entered into a binding agreement, the plaintiff was nonetheless entitled to terminate the defendant's membership for cause, based on his deception in giving a false name, besides defendant misbehaving in other ways.
As echter Name or amtlicher Name sound odd and may even confuse some people, I would advise talking to your project manager or client and recommending that they just write Vorname / Nachname, while noting in the "fine print" that use of a false name is prohibited.
Finally I would think that the foregoing is true not only for dealings in ordinary commerce but also for interactions with official bodies. The U.S. State Department's
Online Nonimmigrant Visa Application (DS-160) (EN) asks for "Surnames" and "Given Names" but does not specify "legal" or "official" on the data entry form. And the application form for a visa to Germany (DE, PDF) likewise asks only "Name (Surname)" and "Forename(s) (Middle Name(s))". Of course, both forms do note that applicants must enter only truthful information.