It's all joking lah!
The translation of a nation name is often chosen according to its sound, but Chinese characters have their own meanings as well, so you can indeed explain it somehow that way, which has nothing to do with the characteristics of that nation.
英国 Britain：英勇之国 the nation of bravery
法国 France：律法之国 the nation of law
德国 Germany：美德之国 the ...
Other than 天真, 单纯 is another word to describe people are innocent, and depends on the context it may imply the person is too naive that it actually seems silly.
(你）真令人捉急呀！ is also a way to say to someone silly(because of his clumsiness or slow) that make you frustrated.
To express “finish remaining work on hand” can use 收尾 in mandarin. And I think this is the word sounds closest to 手尾。So for 我執埋手尾就會走㗎啦, you can say 我做完收尾工作就走啦
唔好咁冇手尾 can be expressed as （做事）不要有头无尾
唔好要人跟(你)手尾 can be expressed as （做事）不要让别人帮你善后（or “擦屁股” ）
整傷脊椎骨就手尾長啦 probably has no similar expression.