Is the best way to understand this construction in the context of this lesson to think of the sentence in English as "If Joseph could come, it would be great"? An example of this reversed structure would be good in the lesson.
Conditional clauses in French, for once, parallel exactly their English counterpart.
It would be great if Joseph came with us. -- "would be" is the conditional of "to be", which corresponds to serait in French. And "came" is imperfect of "to come", which is venait in French.
Ce serait génial, si Joseph venait avec nous. -- It would be great, if Joseph came with us.
Now let's look at the slightly different English sentence you mention:
If Joseph could come with us, it would be great. -- Si Joseph pouvait venir avec nous, ce serait génial.
"Could come" is grammatically ambiguous in English because "could" can be the imperfect as well as the conditional of "can". In French, though, pouvait clearly is imperfect (the conditional would be pourrait). In French, as a rule, you never use the conditional in the si-part of conditional clauses, only in the the main clause. This rule also exists in English (never use "would" in the if-part) but sometimes the rule gets broken anyway and other times, people get confused by the ambiguity of "could".
This translates to "If Joseph were ("venait" imparfait in the French) to come...."
So I'm not able to fully agree with your proposal.
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