2) He could' have been making dozens of friends in the Netherlands by now, but he wasn't into the language.

It's very rare to write "could've" even if that's how we say it.

7) Canould she have changed her outlook on meat?

1) You know, the bullet could've reboundicocheted off the ceiling and penetrated his eye.

Rebounded is not quite the right word here although the meaning is clear.

"Ricochet" is used for projectiles (especially bullets).

"Rebound" is mostly used to describe a ball bouncing off something (mostly in basketball). Otherwise it tends to imply a kind of "bouncing back" as in a recovery. For example, we say that "the economy rebounds after a recession" and we describe a relationship that one enters shortly after a breakup (normally with the implication that it is not expected to last) as a "rebound relationship".

2) He could've been makingmade dozens of friends in the Netherlands by now, but he wasn't into the language.

Use the simple past rather than the progressive past since you define an endpoint (by now).

Alternatively, if you want to use the progressive past, go with: "He could've been making dozens of friends in the Netherlands, but he wasn't into the language."

3) James could hardly have assumed what my intentions were.

This sounds oddly formal and like a quote from a nineteenth century British novel (at least to my American ear). "James could hardly have known what I intended" or "James could hardly have guessed what I intended to do" would sound more natural.

7) Could she have changed her outlookmind on meat?

This may not be wrong. but I am guessing you that want to use mind instead of outlook. I (American) use "change one's mind" for normal changes of opinion - e.g. she used to eat meat but, after watching a documentary, changed her mind about meat and is now a vegetarian. To change one's outlook is rather...grandiose and normally used for large philosophical shifts - e.g. "In a 'Christmas Carol', the wealthy but hardhearted Scrooge learns to see the errors of his ways and changes his outlook on life to be empathetic and charitable.

9) This rule cwould've worked on truants if the principal hadn't removed it.

Use would have if you are asserting it to be true.

1) You know, the bullet could've rebounded off the ceiling and penetrated into his eye.

“Penetrate” already carries the connotation of “into” so this seems redundant.

3) James canould hardly have assumed what my intentions were.

7) Canould she have changed her outlook on meat?

3) James canould hardly have assumed what my intentions were.

7) Canould she have changed her outlook on meat?