Any suggestions for addons/techniques for keeping up LB on three targets over the course of a boss encounter with a lot of other things going on? I'm thinking specifically of fights like Halfus. I can use focus and Clique to monitor and easily reapply to one additional target, but I don't see an obvious efficient way to do this not involving retargeting for a total of three targets.

You might want to consider using a mouseover macro to apply LB, and a tooltip addon that displays debuffs over mobs' heads. There was a discussion about this when WLK was released or maybe even around early Ulduar, where Manly pointed out that mouseover is cancelled by holding in your right mouse button (mouselook). So his playstyle was to hold right mouse to be sure to apply LB to his main target, then let go and mouseover to apply to other targets.

As for keeping track of the three LBs, a simple counter (like the one included with Mage Nuggets) should do the trick. If you can remember the order you cast them, it's all you need. Otherwise, you could use your target, focus, and matter of elimination to determine which enemy lost his bomb. If the targets are all in your field of view but spread out, the explosion makes a pretty recognizable visual also, which may be the easiest way to recognize which bomb needs to be refreshed.

I discovered Fortexorcist while browsing the warlock forum, I can easily see on which target my Lbs are on. You should try it.

There are really a lot of solutions for this--Tidyplates, DocsNameplates and other addons will show debuffs on nameplates whereas I find it easier to track LBs with Scorchio2. Fortexorcist is useful too, but Scorchio has separate bars for your main and secondary targets. On a 2-target fight like Ascendant Council focus frames are nice too.

When I need to keep LBs up on 3 targets I click targets and use a regular bind, and I don't usually lose any DPS time picking targets between GCDs. I don't like mouseover macros because nameplates move around and if you hit the wrong target with your LB bind, you've basically lost a whole GCD, whereas with left-clicking you can just take a quarter second and pick the correct guy.

I think that I know the answer to this, but I am not 100% sure.

When calculating all the variables that go into "hit" with the RNG serverside, does the calculation ignore the decimal remainder after the solid number, or is it factored in.

Example:

If a Mage has 16.85% hit, does the server see the .85%, or is it rounded down to 16% giving a 2% natural chance to miss?

Thanks.

It's not being rounded.

The only stat rounding which affects mages I'm aware of at moment is that you need 517 haste rating rather than 516 to reach the dot soft cap.

The only thing I can imagine is you looking at your melee tab instead of your spell tab.
However, you only need 6% hit for heroic bosses since they are level 87.

In regards to shared HP-pool fights, specifically Omnitron Council and Veralion & Theralion, is it a dps increase to maintain LB's on both targets (with obvious recognition of mechanics, particularly with the shield mechanic on Omnitron)? Or a complete mana sink in the sense that you're wasting a GCD on another LB cast in addition to the mana cost of LB?

Yes, it is a DPS increase. Living bomb provides the 2nd highest damage per cast time of all your spells, right behind flame orb (source: Simulationcraft Results). It would be optimal to maintain LB on both for the entire fight.

The game doesn't round off your fractional points in mastery. The only stat it does that with is haste where we believe it rounds off to the nearest with regards ticks, durations, and cast times (possibly also to the nearest millisecond).