You need an article in the sentences about the Company's seats - where are the companies' seats? (if you are talking about seats belonging to more than one company) or "Where are the company's seats?" (if you are talking about more than one seat belonging to one company).
"To belong," is a verb which implies a permanent state of affairs, not something temporary. So belong is a non-continuous verb and cannot be used in any continuous tense. So you cannot say "This is belonging to." You have to say 'this belongs to.'
"This money belongs to a trade union worker." Because it is very well known that workers belong to trade unions, you do not need to use a possessive in this sentence, hence no apostrophe. Equally "This money belongs to trade union workers." We would probably be more likely to say "This money belongs to a unionized worker." "Unionized," means that they belong to a union.
In the address sentences 's is correct.