Hi Para,
Repotting is generally performed in the spring (on the vast majority of tree species) as it is less stressful and is shortly before all the new growth starts. As repotting involves pruning the roots, sometimes substantially, trying this in summer would be too stressful on the tree and it would most likely die.
Mallsai is just a term many people use to describe mass produced bonsai trees that are often found in chain stores in the US and shopping malls (hence the name). For those seriously into bonsai or those comparing them to trees that are in refinement, mallsai are viewed as more potential bonsai material. I guess another term often used is "Potensai" or "Potential bonsai". (Yup we make up loads of words round here
)
All trees that are raw material are potensai, although some lend themselves better to the art than others.
What you have falls into that catergory and is nothing to worry or be disappointed/ashamed about at all! I have them, as do many people. They are a very good starting point for people to learn about caring and styling of trees. In a few years, with some good growth and styling, this tree will be totally transformed. When I get the chance, I will take a picture of what mine looks like after just 8 months of growth in a training pot.
As you read more and view some truly great trees, your perception of "what a bonsai tree is" will most definately change. When I first started, I thought that just about any tree in a pot was a bonsai (although by literal translation this is true). Now I catergorise them into classes of raw material, potensai, trees in training and those in refinement, etc, etc.
Hope that helps.
All the best,
Aaron