No. The Bubble-Head Charm is only intended for the caster. A similar spell is the Ebublio Jinx which is intended for targets rather than the caster themselves.
You can use transfiguration on ebublio, but you would also be transforming the target with the bubble. To escape the bubble you would want to transfig the bubble into something that's easy to break out of, or simply vanish it. Any spells used inside the bubble will impact the bubble, such as using flipendo. Using this inside the bubble would launch the bubble at the target.
All spells available will be listed on the Catalogue of Books and Tomes page, but this is OOC information only. Over time more spells will be added. ICly you will only be able to learn spells you find in books and other locations.
Reparifargeis is a spell used to reverse unsuccessful transformations. To reverse a successful transformation you use the same spell, as it works both ways. Example; to reverse object to plant you would use plant to object.
Protego Maxima is not a effective counter during a duel. Protego Maxima will conjure a magical protective barrier to cover the area, spells cast towards it will explode. As the spell takes time to conjure, the spell cast towards you would be within the barrier thus no defense is made and why it is not practical for use in a duel.
No, you cannot transfigure a magical animal into an object.
Finite won't work on transformations, conjurations, and forbidden curses. Finite will neither end confundo as that is a mental state.
Any spell that makes the target DO something, or have an effect on will often wear off. Spells that make alterations won't wear off, and will need a counter spell.
You can't transfigure a human into another human, however you can transfigure yourself to take on another's appearance. That being said, this is an extremely advanced type of transfiguration.