yes she could apply for a visit visa, doesn't mean she will get it, you would probably have to sponsor her, but its worth a try.
it cost a mere £67, but prices increase 10th of April
she can spend upto 6 months in anyone trip in the uk, but shouldn't spend more than 6 months in any one year in the uk
nope should not effect her application for a fiancée visa whether she gets the VV or not.
