1> No, you don't have to test it after each little change. That's what log files and FireTuner are for. You can make hundreds of changes at once, and then see which ones cause error messages. That isn't to say that things can't still go wrong without doing this, but those sorts of mistakes would be the ones you'd miss even going one change at a time.
2> No, you don't have to restart the entire game. It might not recognize a rebuilt mod right away, but build it again and it'll usually add it to the list in the Mods menu. Sometimes it might take more than one rebuild, but you'll know when it works because there'll be a message in FireTuner when the mod list changes.
3> Even when you do decide to restart the game (say, to clear the cache) you CAN skip the intro movie. I've never had it take longer than about 30 seconds to load a game from scratch.