1. Replacing everything can take your car out of commission a while especially if you find that you don't have the tools you need or aren't comfortable fabricating your own. If you rebuilt everything in the rear & leave the front alone, looseness in the front may be more evident - when everything is loose, it all tends to "blend" together...
2. The subframe bushings are NO big deal. I have done them, twice in fact (on two different subframes). For the first subframe, I did one side at a time with the subframe in the car. For the second subframe, I removed the entire subframe from the car. No because I had to, because I felt like it. The bushings CAN be removed without special tools. I busted the seals on my floor jack in the process. Installation can probably be done too.
3. There is NO reason to relieve pressure on the springs if you are doing subframe mounts. Only if you are replacing the support joint in the bottom of the wheel carrier. Or, you are replacing the lower control arm or the spring/spring pads. There is very little pressure on them, so the danger is low. However, I did have the spring compressor when I did it. Removing the rear spring w/ the compressor still requires removing the lower control arm at one side.
For MANY models the left and right subframe bushings are the same. However, there are some models where they are not. And some modesl have higher bump stops than others. I recommend calling your parts supplier with your VIN# ready & if they tell you one size fits all call somebody else. The origianl parts were solid rubber & the newer ones are hydraulic (oil filled). Be sure to lubricate the bushings either with the MB sliding fluid or spray them with as much silicon as they will take. Otherwise, they will NOT fit properly (in the front).