The trims on the door panels can be taken off although they are not screw tightened. At the back of the door panel you will see some pins with rectangle or round clamps on them. Both clamps have to be bended to get them off (the rectangle clamps are easier) and once they are off, you can straighten and use them again.
Normally melted plastic pins tighten the armrests. The plastic armrest comes with pins that stick through the door panel and are warm pressed to get secured to the door panel. If you take the armrest of, these plastic pressings have to be cut and you will have to drill a small hole in the middle of the pin and use a screw with ring to tighten the armrest again.
I presume that you have a leaking power steering (return) hose...Normally (9 out of 10 times) you don't need a new hose to solve this. Just unscrew the hose clamp, push the hose a little further on the power steering pipe and fasten the hose clamp again. This will do perfectly in most cases.
The W202 C class (without steering column adjustment) actually has not enough space to get out the dash cluster. If you take the cluster out carefully and use a thin (metal) strip to guide the clusters bottom over the back of the steering wheel and gently get the cluster out of its stuck position between dash and steering wheel towards the right, you CAN get the cluster out without removing the steering wheel and without damaging the dash...
Normally I leave in the steering wheel and remove the lower dash panel to push out the cluster from the back (instead of pulling it with the hooks).
If you want to remove the steering wheel, you need to remove the panel under the dash also (to stick a tool in the back of the steering wheel to untighten the air bag).
You do not need to disconnect the battery before removing the steering wheel. You only have to take care NOT to turn your ignition key after the airbag or the dash cluster is disconnected. It will light the SRS bulb, which has to be reset by the dealer.
So you can take out the airbag and the dash cluster with the battery connected and the ignition key in 0, but you have to take care NOT to switch the ignition key when either of these two parts is disconnected.
Always change ALL 4 dash illumination bulbs (otherwise you have to do the same thing next week again).
The front window gear mechanism (the vertical shaft) can be maintained with any kind of oil; no special requirements.
About the C pillar panel, vinamg has described how to remove that although you don't need to remove the seatbelt bolts.
The 3 screws in the backrest are for the 1 piece backrest. If you have the 4 pieces down folding backrest, you only need to take out one screw of the (left) corner and lift the piece to get it out.
with a minor 600+k