Would it be the same if I replace ''that'' by ''who'' in (3) and by ''which'' in (2)?
In practice, definitely yes. But some strict users of the language believe that you must not, because these clauses are implicitly "restrictive".
The number of people who died in the accident has increased. I think ''who died in the accident'' relates to ''people''.
Yes. And, again, some believe you MUST say "that" here.
In ''degree of force used.'' ''used'' is for ''force'' or ''degree''. ''amount of money given.'' ''given'' describes to what? Same question for ''owed'' in ''duty of care owed''
You know, in practice the participle applies to the "X of Y" clause. And so one could insist the participle applies to X or to Y but if the meaning cannot be clearly distinguished, what does it matter? --Unless, that is, you have a teacher who has a very definite point of view. In which case you should ask your teacher and, for the purposes of the class, follow the advice!