hmm tanking is very important, as it saves everyone on stims and as important as engie healing, miner, guardian and rebel all need good tanks. Every turret or flower that targets an engie or op hurts them more. The logic of tanking is to get hit and lose health on behalf of the engie and op, so every bit makes the team faster and more mana efficient. If there is a lot of potters, for example an engie or op who cast four skills in rapid sucession, then taunting wont work too well, but instead of using all the taunts together, use one at a time, spread out to every single time the boss changes his attention to someone else.
Now about your build, you have a good mix of buffs, skills and damage skills, you have a particular way of using them and thats cool, and yes, there needs to be some kind of co-ordination in the group about the action skills. You dont need to put singularity on 6, even a 1 helps, used very sparingly, as rarely hopefully as engie uses revive. Apart from taunting, another important thing a commando has to do is control the battle. So gravity well and singularity are vital in getting mobs where you need them, and singularity is also a last ditch effort to save someone being hit if taunting doesnt work.

About Nebulous, its only helpful if you go to to toe with enemies, otherwise skip it