Yes, people can use more than one system of magic as long as they have a great connection to both forces that govern them. But they won't be able to access more powerful spells as they advance in each system, because at some point they will reach magical excess where they can no longer hold enough power. Power can be too much for one person to handle.
Sometimes, yes. Arcane Arts uses a source of energy called Willpower, which is sort of what keeps a person vital and strong both physically and mentally. They can only use so much Willpower in one day or they will start to become tired and discombobulated, or even die of exhaustion. Sacred Power relies on constant devotion to faith and the ideals of certain gods and other powers. Losing that connection means losing the magic. Druidism involves overexposure to the more natural side of the world, but staying too long in a civilized and technologically advanced environment will start to deteriorate your magic. Occultism also fuels off of the caster's sanity, their or other's blood, or even life force.
Sometimes overlapping happens with some spells being shown in different traditions. Arcane Arts focus a lot on enhancements like Sacred, research and otherworldly power like Occultism, and also transformative & elemental spells like Druidism. Druidism can also have healing, enhancements, and knowledge inside of it. Sacred allows elemental and transformative powers like Druidism. Sometimes healing can be used by Occultists.
Arcane Arts are gained through study, practice, and hard work. Sacred Power is given by divine beings to people who have devoted themselves to those great beings. Occultism is granted when a person starts to understand the darkness in the world and draws on it for the betterment of others. Druidism is somewhat like Sacred Power but is gained through working in the community and respecting the natural laws of the universe. It can also be drawn from ancestry.
They all mainly use Willpower to cast spells because that energy is a part of all other energies.
They have been used countless times in wars and conflicts throughout Teroth's history. Occultism is seen as an evil form of magic that should never be practiced unless it is a last resort to end tension. Arcane Arts are fairly popular but mostly used by people in the military, academics, or guilds. Druidism is not seen as inferior to Sacred Power, but very ancient and older than Sacred. Sometimes Druids and Acolytes (which is the term for Sacred users) work together in some communities, especially lower-class towns.
Hope this answered all your questions! :-)