Sir, forgive me, but i humbly disagree to some of your points. I'll address them one by one.
Solenoids: Yes engagement of gears is via solenoid. But do you wonder what does the solenoid do in effect?? Once the control unit engages a solenoid, the solenoid provides ON-OFF to a flow of high pressure transmission oil to respective clutch. These are circular band clutches that hold up various parts of a planetary gear set based on required gear ratio.
The aggression of shift and clutch engagement is controlled by another pressure regulating valve that is common to all shift solenoids. In some cases, 02 regulating valves are used.
Solenoids cannot be repaired. They are replaced. Like the same way a dead relay is replaced. Re-winding is not done on small scale.
Coast Clutch: Apart from clutches in gear box, auto transmissions have 02 more clutches. Torque converter lock-up clutch and stator lock-up clutch.
The first one is used to avoid constant slip loss during cruising speed, lock engine to transmission during engine braking (or coasting as you say).
The second one is to lock stator inside torque convertor. At pulls from standstill, stator is locked to increase torque (you have to go through fluid dynamics in a TC to understand how torque is increased). At cruising speeds, the stator is allowed to rotate with TC (this is before enagement of TC lock-up clutch) to reduce fluid drag as TC input and output are starting to match up.
Putting Car to N: It is not recommended. In panic situations, one would be more safe while P is engaged. This will also prevent vehicle movement if one lets go of the brakes. The parking prawl may not be replacement of a hand brake but its there for a reason.
But why would such a panic situation occur on everyday commute??
Driving without OD:
I have yet to grasp why not driving in OD will damage clutch?? You are just working on first three gears. It will in effect would not engage the clutches required for 4th and 5th gear?? Would they be safe or would they be damaged?? I would appreciate a detailed response in this regard.
N close to D:
No, they are not closely placed on PRNDL shifter for such reasons as to iterate between them easily. N is strategically placed between R and D so as to prevent accidental enagegement of reverse from D.
I quote wiki for authentication
" Vehicles conforming to US Government standards[SUP][2][/SUP] must have the modes ordered P-R-N-D-L (left to right, top to bottom, or clockwise). Prior to this, quadrant-selected automatic transmissions often used a P-N-D-L-R layout, or similar. Such a pattern led to a number of deaths and injuries owing to driver error causing unintentional gear selection, as well as the danger of having a selector (when worn) jump into Reverse from Low gear during engine braking maneuvers"
