Ask Mr. Smarty Plants

Even though your question is accompanied by plenty of detail, it is difficult to give you advice without seeing the plant or photos.  You may get more valuable advice by contacting your County Agricultural Extension Office.

That being said, you mention that the plant is "mature" and that it was "in the ground".  Luckily, it is quite young (it will be 15 to 25 feet tall and 10 to 20 feet wide when it is) and it sounds like it is actually doing quite well.  The root system of a tree and what is above the ground are in balance ... the top will not grow to be larger than the root system can support and the root system cannot grow without the food supplied by the photosynthesis done by the leaves.  When it is dug up to be transplanted a significant percentage of the root system is lost and there is no longer enough to support the top.  It is normal for the first few years to have foliage that is smaller and sparser than normal as well as fewer flowers and fruit.  Also, if the tree is drought or heat stressed in the first season it may drop the foliage or fruit it feels it cannot support.

You mention very tall, straight vertical branches.  These may be what is known as "water sprouts" which are common on all types of apple trees. They take away from the beauty of the tree's form and do not bear flowers or fruit.  This website has pruning information you may find helpful and this one has diagrams and a video.  You could also inquire at the nursery where you purchased it.

The only real advice we can give you is ... be patient.  It will take about five years for it to start spreading and taking on the form that make crabapples such popular trees.