I can relate to you on a couple points. I frequently am unable to fly to cornfield-viewing heights. This is most likely because our brains don't have a very good idea of what very high is like. Try practicing flying with a bird's eye view instead of forward. My guess is that when you focus too much on forward, you lack height. I could be wrong.

As for the limitations on other dream powers, I say that it's about your own anticipation. It can be hard to overcome this, and requires patience and flexibility.

Epic dreams, you say? I've had reoccurring characters and situations (my most recent was Birdgirl), but I'm not lucid all the time in these (consequentially, I only get the story moving when I'm lucid). I was surprised when the characters first started reappearing when non-lucid. I've had an adventure that spanned for months, with only a few episodes that dealt with the actual topic (the rest was non lucid rubbish), and one that lasted a couple weeks (with mostly reoccurring content and characters). Not exactly like your experience, but relevant. I see no reason why you'd become ill from these dreams, except that you may be subconsciously inflicting it upon yourself. Dreams are not supposed to be physically draining (indeed, you are paralyzed during them). Perhaps these dreams are correlated to the weakness by a more logical factor, such as a minor sleep disorder.