Trying to find time to play...
1) Some cards give you X points when game ends e.g. trailer bonuses. Do you have to keep these cards in your hand and not play them until end game?Yes you have to keep them in your hand in order to get the prestige points
2) Some cards give you one point when game end. e.g. extra resources. CAn you get the extra points even if the card function is used?These cards have 2 uses. You use them to get extra resources, or if you don't use this help then, at the end of the game if these cards are still in your hand, you get prestige points.
3) Most of the games we played, we seldom keep more than 2 investment cards in our hand, is the case in your games? I don't see the use of magician keeping an extra card in handIf you have magicians then you can keep many investment cards in your hand so as above you may earn some more prestige points