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Yes, those clauses are relative clauses, without doubt.

No, it is not possible to change "which" to "that", the reason being that this relative clause being a non-defining relative clause it can't retain its non-defining property if "that" is used. Let's recall that the comma is the mark of a non-defining clause, otherwise called a non-determining clause or non-restrictive clause. https://academicguides.waldenu.edu/writingcenter/grammar/clauses

No, the clause in which "which makes" is replaced by "making" is not a relative clause. It is a nonfinite clause without subject. However, the nonfinite verb form ("making") is a formal indicator of subordination, just as the relative pronoun (in this case, "which") is such a formal indicator. The comma is not a part of the clause: it is a sentence element with the purpose of articulating the sentence. It can be considered as a marker of syntax, this being so because the syntax is different when it is absent.