Well, firstly, is the port working or not? You can try glueing that ring back in place, another option would be buying a USB card which could allow you to connect the mic through a USB port, or just use the ports placed on the motherboard, instead of those situated on the PC case
The problem can be solved by reinstalling the MacOS from Apple Recovery Menu. It looks like that SSD of yours has the settings of early 2013 model, which are conflicting with your late 2012 Mac. Yes, you’ll lose all your data (just do not forget to backup, or move to another drive, all important files, before reinstalling the OS), but you’ll have a normal working machine.
Problem resolved. The problem was in the console itself. The fuse which was responding for controllers’ work was blown, so I changed it, and now it is working smoothly
On the lower part, where’s placed the stylus, on the left side the should be 2 or 1 slots ( depending on the fact, if you bought the model with SIM card or not), lift the door of that slot, and insert the SD card as shown on the door’s pic
Yes, order another UMD unit for your model, and take apart, the console, until you reach the motherboard, take of the mother board, detaching all the ribbons, and after take the old UMD drive out, and install the new one, then reassemble all back.
Well, if you mean that the faulty buttons are only the ones, under the display, than the easiest way, is to replace the flex cable with these buttons from the bracket. You can find sets with all three flex cables (the one for buttons under the display, the one for D-pad, and the one with the functional buttons) on AliExpress for like 2$ , and you can replace entirely all 3 ribbons for avoiding over problems.