LESS is a very good idea to use, makes Kendo styles much more readable! However, right now the LESS syntax used to generate styles (ThemeBuilder for example) use a non-official branch of LESS syntax. Unfortunately this makes LESS impossible to compile any other way than using the changed JS libraries and harder to integrate with other LESS in the project. I suggest you either convince the LESS community that the changes are good and merge them into the official branch or consider the possibilities to use official LESS syntax.
We managed to switch to the official version some time ago: https://github.com/telerik/kendo-ui-core/blob/master/package.json#L20-L21
Sounds promising! What has changed since last time? As I remember it there were Require statements that were not supported at that time. Have those been replaced now?
You should have no trouble compiling Kendo UI Web LESS with normal LESS build tools. Please make sure your LESS tools are up-to-date, though, as Kendo UI does leverage some of the newer LESS language features, including guarded expressions: http://lesscss.org/#-pattern-matching-and-guard-expressions With an up-to-date parser, you should have no trouble compiling Kendo UI Web LESS themes. There is some custom LESS syntax used by Kendo UI Mobile, but we are looking now at our options for avoiding that custom syntax.