The packages are usually put into the repository for the next release around the time the Ubuntu release happens (in Mint 14's case, a week before). Theoretically, you could add packages from this repository and from the linux mint github stuff, but there is no iso published until the RC. Note that for Linux Mint, the RC is considered more of a public beta than a candidate for release that won't have an changes unless there are major bugs - there can be many changes between RC and release. As for updates, Linux Mint recieves the same updates Ubuntu does to most applications. If Ubuntu fixes a bug, then the bug is fixed in Linux Mint. However, for mint software like mintInstall and mintUpdate, updates are pushed whenever they are ready. My guess is if there was a major bug that Ubuntu didn't fix, then linux mint would add their own patched version, but I haven't seen that happen so far.
Dell XPS 15 l502x - Debian Testing 64-bit NetInst Xfce, SolydX 64-bit Debian Testing, SolydK 64-bit SolydXK Testing
Old Gateway Pentium 4 Desktop - Arch Linux 64-bit Xfce and SolydX 32-bit Sid