Congratulations on your engagement!!
The first thing you need to do is talk to your wedding planner! A good wedding planner can answer all your questions and calm any fears you may have. If he/she can't do that, you may want to consider finding a new wedding planner.
The first think you need to do is have your bridal party get their attire in order. As for flower girl dresses, you don't have to go to a bridal shop if you want to cut on cost. Try a department store for matching dresses for the girls. Since the holidays are approaching there should be quite a few to choose from sticking with your black, red, white theme.
Having someone do an inexpensive cake for her to use in her portfolio is a good idea...just make sure that the cake not only looks good, but tastes good too!
Your mom might be right when it comes to the stationary, that is something you will need to price out to determine what is more cost effective. Doing the flowers yourself is not a bad idea, but might be time consuming, make sure you plan enough time to get everything done. Research local places that sell flowers wholesale, you should be able to get a better price. They can be as simple as a 5" high round or square vase in the center of the table, use floral foam inside the vase and just make sure that you pack in the flowers leaving no empty spots. And make sure that there are different levels to the flowers, some a little taller than other. Then just sprinkle some rose petals on the table and you have a very elegant centerpiece.
As for the ceremony, it really can be as long or as short as you want. Music does not need to be playing the entire time, but you can have a special song played maybe during the lighting of the unity candle if you would like (besides the music you will walk down the isle to).
Like I said, this is all stuff that your wedding planner should be helping you with.
Please feel free to contact me directly if you have any other questions.
Leanne