You didn't say whether your students are native speakers of English, nor whether you are.
It makes a difference.
A native speaker of English would, by saying the sentences aloud, realize that, while not all of these constructions are equally frequent in native speech, they are all perfectly ordinary and grammatical, with past, or perfect, of all varieties
... provided that ...
the events reported in the past perfect happened before everything else.
A non-native speaker of English, however, may have been taught that there is a strict rule called Sequence Of Tenses that governs every clause in every sentence, with terrible (but unstated)
penalties for violation. This is of course not true, but many wish it were, and try to enforce it.
For that matter, most native English speakers are taught this kind of mythology in school, too,
but luckily it's usually ignored, except by the credulous.