In my experience, in Southern China it’s actually the other way around. The “h” is often dropped (not just in “sh” but also in “ch” and “zh”). So I agree that 4 (sì) and 10 (shí) can sound similar in parts of Southern China, but it’s because they are pronounced as sì and sí, respectively.

Sometimes we do say: 这事儿 rather than 这件事，becasue 这事儿 looks more spoken.

这句子 in theory is 这个句子，we don’t say 这句句子(aloght it’s correct)，becasue this put two “句”s together, will make it strange.

I have heard measure words left out in clear contexts: 我把这事忘了, or 情你把这句子写下来. In theory it’s 这件事 and 这句句子. I am talking of spoken language here.

Measure words sometimes are crucial to determining what someone’s talking about, think of the difference between 一套房子 (an apartement) and 一间房子 (a room).

It’s not correct: wǒmen shì sān rén 我们是三人

But the following sentence, both are correct.

母子二人 is short for 母子两个人

母子二人相依为命。=母子两个人相依为命。

Honestly，hourse later.两个小时后 or 两小时后 ， both are same to me，no different.

Sometimes I will say 两个小时后 , sometimes 两小时后.

e.g

When should we met there? Two hours later.

我们什么时候在那儿见面？ 两个小时后 / 两小时后。