They are aimed at holding him back from doing drugs.

In my opinion this is the only one that's a little unnatural. I can't really explain why but 'hold back' doesn't fit here. I would never use 'hold back' here but instead would use 'prevent.'

Tell me if they sound unnatural and how you would you write them if you don't mind :).

The government held back information on dreadful affairs,; a case in point would be the death of a stowaway who was clutched toing a boat’s propeller.

You cannot hold back affairs, only information about them.

We also don't have the phrase "was clutched to"; the verb "clutch" requires an actor, who in this case would be the stowaway.

Unless we hold back negative emotions, won’t wee won't succeed in it!

Her fixation with minimumute details will hold back the development of the projects, for such aand for this reason, after deliberating ithe matter, I hoeld back from participating in them.

In the phrase "after deliberating it", it is unclear what "it" refers to, so I have replaced it with "the matter". Alternative, you may write "after some deliberation".

Should the police force have to hold back the riots back, it would have no precedent.

Unless we hold back negative emotions, won’t wee won't succeed in it!

Her fixation with minimum details will hold back the development of the projects, for such a reason, after deliberating it, I hold back from participating in it.

We have seen a surge of policies which have held funds back for universities or research.