1) It sounds like you've got the wrong bag delivered.
When I ordered my Serenity in 2007, the Serenity 2007 board bag is indeed perfectly shaped and tailored to fit the Serenity hull. Still in great conditions after all those years of intensive usage, no complains.
2) To make things clear, which model of Serenity do you have ? Mark I (2007-2008) or a Mark II (2009-2012) ?
Which board bag did your dealer order ? A bag for a Mark I or for a Mark II Serenity ?
Which board bag did you get delivered ? How long in cm does it measure in lenght ?
3) Testing the Serenity is usually done via the distributor of your country because he knows to which shop the board has been delivered to. You then just have to get in touch with the shop in question to demo the board of interest. Or get in touch with a school/shop that has a Serenity for rent. But I agree she's not available everywhere for testing because this is the decision of the distributor or the shop owner to let you try one or not...
PS: Serenity Mark I :
Serenity Mark II: