For a definite answer, Bob should ask his tax advisor.
German freelance status ("Freiberufler") is a bit difficult to navigate, because legally speaking, this status can only be applied to contract work that requires a university degree, everything else is a regular business ("Gewerbe") that is taxed differently and requires you to join ...
Not on its own, no
A ToS can be of some value, but will not fully protect the site operator (host). For one thing, a ToS is probably not binding on the user based on a contract of adhesion such as "by using this site you agree to...". A specific act, such as clicking a checkbox that defaults to clicked or activating an "I accept the ...
Short answer: Yes, but with some limitations - none of which are age-related so do not restrict a citizen's arrest to adults only.
Long answer: The relevant legislation is at s.24A of the Policeand Criminal Evidence Act 1984 which begins with:
(1) A person other than a constable may arrest without a warrant—
Which is followed by the ...
Article 10 (1) Basic Law / Art. 10 Abs. 1 Grundgesetz:
The privacy of correspondence, posts, and telecommunications shall be inviolable.
You can send whatever you want via postcard (as long as it doesn't constitute a different criminal offense).
Technically, whoever were to decrypt your message, besides the addressee, would probably commit Data Espionage ...
Is this self defence?
German self-defence laws may be found in within Chapter 2, Title 4 of the German Criminal Code at sections 32 to 35:
Section 32 Self-defence
(1) Whoever commits an act in self-defence does not act unlawfully.
(2) ‘Self-defence’ means any defensive action which is necessary to avert a present unlawful attack on oneself or another.