Normally the back teeth do not touch just when you complete invisalign. The plastic is just as thick in the back as it is in the front, so when you remove the trays and bite, because your jaws are a hinge, the front move further during the motion, and thus contact first....
An underbite, also known as a Class III Malocclusion, can be the result of an underdeveloped maxilla (upper jaw) or overdeveloped mandible (lower jaw). Severe cases almost always require orthodontic surgery (orthognathic surgery). However, mild cases, such as yours do very well with simple...
Invisalign and braces in general work best when a constant force is applied to the teeth causing tooth movement. If you are not compliant in wearing your Invisalign trays constantly then I would not recommend them. Your teeth will be in a state of shifting during the evening and...