hi welcome to the forums
it's best practice to open separate topics for separate subjects
but with that said i think i can try to answer some of your questions
1- yes you can install mint without having any other OS in the computer.
2- i'm using kde for ages and it's the most stable and feature rich DE at this moment (but things vary a lot depending on hardware and distro)
3- mint doesn't have a in-place upgrade like ubuntu; we recommend fresh installation.
4- mint has 2 releases: one following debian testing and one following ubuntu: the last one sees a new release short after ubuntu.
5- same as ubuntu: last lts was mint 13 (based on ubuntu 12.04 precise) released on may 2012 and supported until april 2017. see http://www.linuxmint.com/oldreleases.php
7- lts (specially this one supported for 5 years) are supported for a very long period of time; the only thing we can say is that for now you won't miss anything, the most relevant improvements (from an enduser point of view were backported from mint14 to mint13, see here >> http://blog.linuxmint.com/?p=2225
8- same as ubuntu
9- mint has its own software manager and also synaptic.
10- mintupdate does neither: it displays a different icon in the panel when updates are available and it's up to the user to start the process.