1.
If you have more than 1 Twitter cards meta tags sets, the last one will be considered by Twitter to be presented in the timelines.
You can change the values of the meta tags periodically, but be aware that Twitter caches the meta tags for some days before refreshing that info from your page.
2.
No, as soon as your domain got approved for a card type, there’s no need to request approval again. You need to request approval for each type of card you adopt in a domain.