While I've always personally preferred SCSS over LESS, when I'm starting new projects these days I tend to just use the LESS version of Bootstrap, as it makes it much easier to update, it's tested much more widely, and fixes for any issues that do crop up are available faster.
I'm not aware of any LESS equivalents (but then again, I do very little LESS work).
It may be heresy, but I haven't had any issues using both preprocessors in a single project, so I'll often just use LESS for Bootstrap-related stuff and SCSS for everything else.
Yes, LESS is actively developed and (incrementally) changing: https://github.com/less/less.js
As for Bootstrap, I have no insider information, but would be very surprised if they abruptly switched.
On the contrary: last year when I asked @mdo and @fat if Bootstrap would ever go mobile-first (vs the horrible mid-sized-first-and-scale-in-both–directions approach from TBS2), I got a flat "no" in response. [groan] So I have no idea what those guys might do in the future. But for them to switch doesn't seem likely to me.
FWIW My current project is using TBS3; so far, so good.
Choose whatever pre-processor you're more comfortable using.