My own advice.
1. Yes, but be careful.. just because something is allowed in less it doesn't mean it is recommended. I try and use the minimum required to make the CSS scalable.
2. I use dot less because
a) I don't have a node environment to run less.is in.
b) dot less is more open source.. I found pull requests to support new css features still awaiting the one person running less.J's to pull them
c) I don't want code to run client side and slow things down.
d) I want to inject variables from c# into less for turning.
e) I'm working on 2 extensions, one for sprites, one for rtl which wouldn't work with less.js
3. Have you looked at the site and wiki? What are you stuck on?
Then your browser can look at the css file while you edit the less.
dotless.compiler.exe --watch <filename.less>