"If you could have driven, I would have lent you my car." Yes, this is correct and natural sounding. In general, you don't let modal verbs into the if-clause, but can and could are exceptions. That's just how they work. Also, because they are modals, it is not possible to create a past perfect pattern with them, so "could have driven" is the closest you can get. It still works because it's a pattern we use for imagined past. If you want to use a past perfect pattern, then remove can/could and replace it with "be able to". "If you were able to drive, I would lend you my car." "If you had been able to drive, I would have lent you my car." If you're comfortable with using the grammar from a technical perspective, then no problem. However, if you are really struggling then you could follow Jen's advice and look for good examples to follow (and accept that the examples you get are simply correct; no questions).

Vale, I can only answer the first part of your question. I would personally not have a problem saying your revised sentence - so at least from the perspective of common usage, it is valid. Someone with more grammar knowledge will have to answer the rest of your questions. Personally, in my own language learning process, I am moving away from worrying about grammar, and focusing more on exposing myself to valid sentences. I find it to be a much more engaging way of studying (thus I am more likely to continue.) I'm experimenting right now with AJATT's Massive-Context Cloze Deletions approach, explained here: http://www.alljapaneseallthetime.com/blog/10000-sentences-is-dead-long-live-mcds All the best!

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