I have a question about certain conditional sentences. Specifically with these rules:
If + present = future For example: "If I win the lottery, I will go to France."
If + past = conditional (preterit form of a modal) For example: "If I won the lottery, I would go to France."
My question is about the use of should with these rules (in American English). Why is it okay to say "If I win the lottery, I should donate to charity," or "If you have the time you should read this book"? Why is it acceptable to use the preterit form of shall if the rule demands the future?
A secondary question: As a native speaker of English, I find it acceptable to use could with both 'If + present' and 'If + past' clauses. Is that wrong, or is it simply a particularity of spoken English?