It is indeed upgradeable, Transcend is on to it. But as long as the SSD-controllers are rare it will take some time.

Flexible ground contact with the rear case. Macbook should boot without them, as it can boot without the rear case.

The left part is only the south bridge. There is no need for cooling it

These are just traces of a lathes tool, not brushed. Maybe made of metal due to stabilization under high temperature (more silent)

No, can be repaired. For example at www.apfel-labor.de in Germany. Did it there, works like a charm now.

Just a software bug. It can't sync with mobile network signals because the date in the phone (due to power loss) is wrong

You need a set of new logic chips and a reflow for them. You must have scrubbed them away. Nothing to do for beginners as the chips have the size of 1x1mm and need special SMD skills.

No 400$. costs about 85€ in Germany. Simply follow the guide where it says "Only use the plastic spudger at the three points shown in the image". If you use it somewhere else, your fault.