1.) I'm not even messing with my gas block. It comes factory-pinned, so I'm gonna leave it alone. If I need gas mitigation for it, I'll probably stick a Bootleg adjustable BCG in it.
2.) Yes
3.) Not really, once the suppressor is over the brake, the whole suppressor acts like a giant muzzle brake, due to the nature of how a suppressor works. But, there are benefits of running a QD muzzle brake...Especially on a short barreled 5.56... The main one, is the muzzle brake helps to mitigate blast-baffle erosion by disrupting the gases and flames to the sides than directly hitting the cone and aperture of the blast baffle. So, it will help your suppressor to last longer. And another one, is you get all the benefits of having a muzzle brake on your gun, when you aren't running the suppressor, like flatter shooting and recoil reduction.
4.) I would recommend just leaving the current linear compensator on there until you get ready to run suppressed, or until this JMac extension is made. The linear comp actually does some good, and they do work for pushing the gases and sound forward. It doesn't seem like it's working because your face and ears are so close to the muzzle, but it IS actually working.

And when you get ready to put the JMac extension on there, I would recommend either buying one of the awesome JMac muzzle devices, or the Dead Air KeyMo 3-chamber brake.