The jacket length really is up to you, usually they are a bit shorter, but yes, more modern jacket might be longer. It is very interesting looking at old photographs, many of those jackets pictured are "normal" sports jackets.
The reason for a shorter back is that it allows the kilt to move to the best, and the rounded front to accommodate the sporran.
Kilt wearing is not an exact science, and sometimes it has more to do with the confidence of the wearer.
As you will read on here, "Traditional kilt wearing" can be fairly creative, with sometimes a interesting mix of fabrics or colours or textures.
It sounds a nice jacket, and if you have a chance please post a photo of it being modelled!!