Learn English – Can we negate ‘heaven forbid’ in a conditional sentence like this: “If heaven didn’t forbid it, I’d sleep with her”

The short answer is that, yes, you absolutely can use "would have" with past simple, just as you know you can use it with past perfect. When you learn English, you may well learn about 2nd conditionals (If + past simple, would/wouldn't + verb) and 4rd conditionals (If + past perfect, would have + past participle). You may not have learned that it is possible to mix conditionals. Oh the flexibility of language! Let's see:

Rule of thumb: We use the 3rd conditional (as mentioned before) to describe past situations that we cannot change, whilst we use the 2nd conditional to talk about situations referring to the present time.

2nd conditional: If I were taller, I'd be able to see over that fence.

3rd conditional: If I had been born a giant, I would have dunked that 3-pointer easily.

So it stands to reason that given the right ingredients (or situation) we can mix up elements of these two language constructs.

To comment on the book quote you referenced, the structure is correct. Trisha was thinking (in that particular moment in the past) (hence past simple) it was impossible for me to turn up, however, as soon as I did, the reference to me not showing up became completely impossible, as shown by "wouldn't have shown up" rather than "wouldn't show up".

Unfortunately, this example

If you called me we would have gone for a movie.

is not right. Since the opportunity to call in that particular instance has passed, you can't use a reference to the present situation, but rather to the missed opportunity, like so:

If you had called me we would have gone for a movie.

Hopefully this has helped you, but I'll present a final example to see if we can really crack the case.

I am in a sweet shop with my five-year-old son. He's a bit mischievous and so with my back turned he tries to grab some sweets from a high jar, which smashes on the floor. In my embarrassed state I think to myself "If he were taller, he would have been able to grab the jar no problem."

I comment on the general situation (my son is not so tall) mixed with the (unfortunately) impossible to change scenario of a broken jar (he would have been able to grab the jar). Hence, referring to multiple aspects of time allow us to use a combination of tenses. Equally, I could have used a pure 3rd conditional:

If he had jumped higher, he would have been able to grab the jar.

Either way, we're not allowed back in the sweet shop.

You don't usually use "both" in the negative. Instead, you should use neither such as "Neither of them went". I don't mean that " Both of them didn't go" is not correct. What I mean is that the use of neither is more common than and preferable to the use of both" in this negative sentence. See the last paragraph of the link.