If you really need to, I've seen people mount 2 bottles behind the seat. Cambelback makes a super-light camelback, and has a bladder that is half size. Even completely full, I don't feel it at all.
I think you've got the right recipe here, and I agree with throwing in the Yeti ASR5. However, after riding them, and if comfort is your issue, I think you'll find that the ASR5 depends on the shock for ultimate dampening, while the other two use the DW magic.
I honestly think at a certain point, there is no 'better' and between Ibis SL and 5.7. Bikes are finally getting that good. Its just a matter of how they feel when you test them. Oh, and if you have strong opinions about carbon vs aluminum. I was just talking to my lbs on behalf of a friend about ironically choosing between exactly those two, and was making the alu vs carbon argument and they said they've sent out a heck of a lot Ibis bikes and none have come back from carbon breaking issues.
Honestly, those are two damn good bikes. The 5.7 is lighter than the 5 from last year, but I think the MojoSL is lighter? But unless the weight difference is huge I wouldn't make the decision on that. If it was MojoSL vs 5, I'd say that the 5 is the better climber and the SL the better descender (we're talking by small margins, but I still think that distinction is there) but with the new 5.7 I wouldn't say that.
Specialized brain front and back is good, but a bit harsh and not as good at drivetrain isolation on small bumps, because depending on the bump it doesn't open. I love climbing on my Mach5 - its like riding on the road in a certain sense. Oh, and some don't realize this, if you're climbing technical stuff both the Mojo and 5.7 will climb better with the shock open as the real wheel can do what it needs to in order to stay against the ground. Better than a hardtail - yes - even climbing, and better than the brain stuff, which does do that, but to a much lesser extent. The brain mimicks a hardtail more than the dw bikes until you descend at which point it does quite well. So, if you want a hardtail on the non-downhills - and many do - then that is the way to go (one of the best bikers I actually know personally uses that, and some La Ruta winners use the exact same Specialized bike.).