Inevitable (Cannot Help But)

Naming the one option that remains.

～よりほかない vs ～ざるを得ない: よりほかない points at the surviving choice ('there is nothing for it but X'). ざるを得ない says you cannot avoid doing it, framing the same situation as compulsion rather than as a last option. The reluctance is in both; only the framing moves.

Something will inevitably produce an effect, whoever is involved.

Verb ずにはおかない vs Verb ざるを得ない: Use ずにはおかない. It predicts an unavoidable consequence in the world (見る者を感動させずにはおかない). ざるを得ない is about a person having no choice but to act, so it needs an agent under compulsion.

Social or moral pressure means the matter cannot simply be dropped.

Verb ないではすまない vs Verb ざるを得ない: Use ないではすまない: 謝らないではすまない. The compulsion is other people's expectations, not circumstance. ざるを得ない points at circumstances that leave no alternative and carries no sense of a duty owed.

Circumstances leave you no alternative but to act.

Verb ざるを得ない vs Noun を禁じ得ない: Use ざるを得ない. It reports an action forced on you by the situation. を禁じ得ない takes an emotion noun (驚きを禁じ得ない) and describes a reaction you could not suppress, so it cannot carry an action at all.