这个句子 怎么翻译才对呢？

建议一：
He remained nowhere to be seen by 9 o'clock in the evening.

建议二：
用 "过去完成时态" 来表达。

据我所知，英语中的过去完成时的其中一种用法，是表达过去的过去。所以，"昨晚的九时" 是过去时态，而在 "昨晚的九时" 之前的时间，便是 "过去的过去" 时态，可以用过去完成时来表达。

换言之，当要表达 "已经出现" 时，可以这样说：
He had shown up by 9 o'clock in the evening
= When it was 9 o'clock in the evening, he had shown up
= 他在九时前已经出现，并在九时的时候，仍然在那儿

所以，当换成相反的 "没有出现" 时，便应该是：
He had not shown up by 9 o'clock in the evening
= When it was 9 o'clock in the evening, he had not shown up
= 他在九时前没有出现，并在九时的时候，也不在那儿
= 楼主想要表达的："他直到晚上九点还没出现"

亦可以再加上 still 一字来强调 "仍然没有出现"：He had still not shown up by 9 o'clock in the evening

至於第 2 句（他帮助那些孩子们，直到他去世），可以考虑译作：
He helped those children until he died.

这个问题挺有意思。
我的理解是
He didn't appear until nine o'clock是说他在九点的时候出现了，跟要表达的意思不同。

他直到晚上九点 还没出现
供参考
We waited until 9 p.m. but he never appeared.

他直到晚上九点才出现He was there until nine p.m.
他直到晚上九点 还没出现He didn't appear until nine p.m.
他没有出现直到晚上9点钟He didn't appear until nine o'clock in the evening
？

楼主的翻译是有偏差的，如楼主所说，中文的意思是9点他还没出现，而楼主翻译的英文的意思是9点他出现了。我给出的翻译是：He still didn't appear at 9 o'clock in the evening .供楼主参考

He didn't appear though it had been 9 o'clock pm.

你翻译的对 He didn't appear（ show up ) until 9 o'clock in the evening