x2 on removing the cover and using pliers to get the broken bolt out. Had the exact same issue - ARB cover, tried using Lube Locker specs = two broken bolts. Pliers got them out easily. If you're really unlucky and the bolt is not sticking out enough for pliers to work there are a variety of ways to get a broken bolt out with drill bits designed for that purpose.
After the broken bolts I searched around on this forum and the interwebs and found most people are not tightening to LL specs and are having no problems. So, I did the same - basically tightening to around 15 then to as tight as I can get them hand tightened with a regular sized wrench and have had no problems at all with loose covers or leaks. This worked on my ARB cover and my Riddlers (which is what I have now).
So, I doubt its an issue with the bolts causing your leak unless you had one or two bolts that were very loose. More likely it was an issue with the gasket (maybe a defect), or it was installed off center slightly (but that would be hard to do), or maybe an issue with your cover or your diff. If its an issue with a gouge or something in your cover or diff, RTV may be the only way to seal it with no leaks. Good luck!
07 Jeep Green Rubi Unlimited, auto
Mods: Tuffy Cubby Storage Cover, Rugged Ridge grab handles, Shrockworks Sliders, Rock Hard Engine / Gas Tank Skids, Rhino-lined tub, Kilby Evap Can Relocation Kit, 16x8 Centerline Wilderness Series - Falcon Rims, 285/75/16 Interco SSRs, OME 2" HD Lift, PSC Diff Covers, Superwinch EPi 9.0, BulldogFab Rear Bumper, RedrockFab Front Bumper and skid