Since the first part was answered, I’m only going to answer the are we/aren’t we question you have. “We are on holiday together, aren’t we?” is correct. The only time “are we” would sound natural in that sentence is if you made it negative: “We aren’t on holiday together, are we?” The reason for this is because the “are we/aren’t we?” part is idiomatic. You want the person you’re speaking with to agree with you, and you want to confirm what you think. The question has to be either positive or negative depending on whether the initial statement is positive or negative.

The first sentence is ok. Our only wish is that no one is/was hurt. We must thoroughly question the reasons (for it). We must thoroughly investigate the causes (of it).

In a way, I think they are called smart phones because they’re “smart.” They have capabilities beyond that of a regular phone, and I think that’s how they got the name

I’ve heard eye boogers a lot more than the other ones

Yes both are fine