Mantra Magic is the Blue Magic spell that switches HP and MP I think.

1) I think it is a very bad idea to have all party members with a similar job at the same time. I like diversity in my party. You don't want 6 mages to run into a battle where the enimies all love to use silence on you. As for the second part, there is no job other then red mage that is specified to mix Physical and Magick abilities. You can always have a physical character equip magic as a secondary ability though if you really want a Black Mage Fighter. Just don't expect the secondary ability to save your party because chances are a character trained in physical classes will be lacking in their magical talents

2) As previously stated, after the final boss you get to take on harder optional missions and get a few more characters who you might recognize from other Ivalice Games

3) Well Mantra magic in itself is a very risky strategy, because while it works wonders to raise MP, you are crippled in terms of MP if you start right away with it. I find magick users better suited for tossing spells at the enemy from a good distance, because a strong physical attacker can take them out in 2 turns given the chance. Blood Price is a really great ability though. Just be sure to have a unit heal regularly if you are using it.