Legal Question in Real Estate Law in California

The deceased's interest in the property automatically passes to the surviving tenant. That's what a joint tenancy is.

Yes, IF the title is vested in "joint tenants with right of survivorship". If it is vested merely in the two of you as "joint tenants" without the "right of survivorship" language, then no. Usually the "joint tenants" and "right of survivorship" language are used together, so most likely the answer will be yes, but you should check your deed.

Yes. The half interest of the predeceasing joint tenant just becomes a nullity, as though it had never existed. Nothing can be passed along to the deceased former joint tenant's heirs.

Unmarried couples who buy real property together are, in effect, in business together insofar as the law will treat their property purchases - in other words, family law principles don't apply unless you register as domestic partners, and your co-ownership, irrespective of how you hold title, will fall under regular contract and partnership law principles.

It is practical, if unromantic, to anticipate the future with a written agreement covering your deal and the parties' rights and responsibilities if something goes wrong..

Even if you remain in love the rest of your lives, at some point one will pass on and the home will belong, under joint tenancy, to the survivor. If that's what you both want, fine. Some lawyers think unmarried couples should hold their real property as tenants in common. It's a personal choice.