I think your test is flawed.
It is the original query– the one you save to $temp— that is important for working out which template loads. You overwrite that and paginate based on that overwritten query. That means that the pagination is out of sync with the original query. When you click a pagination link and then the page loads a query runs that does not have your modifications– 'showposts=2&post_type=recensione'.'&paged='.$paged— and that query is used to work out the page to load. In some cases, probably many, the original query can’t make sense of the pagination based on the modified query and you get a 404.
I don’t know why “Custom Posts Per Page” solves it. I am not familiar with that plugin.