From what I know and read here on GameFAQs, as long as you've at least downloaded the game ONCE it will forever be in your download list, unless PSN/Sony decided to permanently remove it from their catalog due to license expiration, company went bust or something like that. So basically, download once it's forever yours AS long as you stay subscribed.
Secondly - Yes, games are swapped out for new ones, there will never be more than 6 Vita games at once. Games you get for free are playable as long as your subscription is active.
Thirdly - As long as you put any free game into your download list, whether you download it or not at that time, it will still be available for download, in your list, as long as your subscription is active. --- PSN ID: RyuuHou24 "I never said that....and even if I said it, I never said it" - Dr Peter Venkman RGB
Sometimes, yes. But don't worry, as long as it's in your download list, you can play them anytime you want. --- "You better not change the game, and you better be DAMN sure you don't make it the same game every time!" - RE Fans