異論 and 賛成: no objection and active support
Compare having no disagreement about a proposal’s direction with supporting a park-preservation proposal. An absence of objections does not establish everyone’s expressed support.
A differing view on a direction or decision
異論 (いろん) means disagreement or an objection. 「提案の方向性に異論はなかった。」 says there was no disagreement about the proposal’s direction.
「委員の一人が決定に異論を唱えた。」 says one committee member voiced opposition to the decision. That member’s view is not automatically the position of the whole committee.
Supporting a proposal
賛成 (さんせい) means agreement or support. In 「私は公園を残す案に賛成した。」, the speaker supported the proposal to preserve the park.
「全員の賛成を得て計画が正式に決まった。」 says the plan was formally decided with everyone’s agreement. Both collective agreement and a formal decision are explicitly stated here.
An absent objection and stated support
The absence of disagreement about the direction does not say that each person actively voiced support. Do not import collective approval or a formal decision from the separate example.
Do not infer hidden opposition either. Keep the scope of the negated disagreement and separately check whether support or a decision is stated.
Does 「提案の方向性に異論はなかった。」 establish that everyone voiced support and formally decided the plan?
Show the answer and why
No. It establishes no disagreement about the proposal’s direction. Everyone’s expressed support and a formal decision are not stated in this sentence.