A person can improve an aptitude test score with some appropriate training, but a higher IQ person will tend to score proportionally higher given the same training/experience.
Yes, but the tests don't account for a similar amount of training, and so do not provide an adequate comparison. The whole point of IQ tests are to present problems which the test taker sould not have previously encountered, thus measuring their ability to quickly devise a solution, and indicating their intelligence on a standard distribution vs the population in general.