What I would do is fully remove the motherboard from the A1200, completely. Unhitch the floppy drive and keyboard and remove any cards. And then hook up an external floppy drive and mouse and boot and count elephants until she clocked on. Then boot from floppy externally. And while your at it stick a backlight behind the mobo and take some cool pics like XRAY thro the board. Just looks cool. Then take very close up pictures of each and every capacitor for sng's and worry over them in close up on your PC.
The test out of the case with nothing attached will tell you if she is working. Get SySInfo and other test mobo floppies. Not sure I read anything about the PSU. But if you have a light brick get a heavier one. And check the PSU with it unplugged from the Amiga and listen for a low thumping noise. Don't worry wont catch on fire till it rattles very loud. Make sure your power connector hasn't worked loose.
I recently had what you had cus the floppy disk button was stuck. And also press thise switches up and down a bit on the mouth to the floppy.
Just start from the ground up and that starts at the power plug. Check wiring PSU connector stripped motherboard dry joints rear view and xray and capacitor. Its probably the floppt ribbon loose or the power switch or even a broken pillar on a key that is sending a delete signal continuously to the computer. By removing the possibility for faults you get to the problem quicker.
You didn't spill coffee on her did you... ? Retrace what you did. And don't leave heavy objects on the keyboard again... That book was way to heavy.
There is no point getting something fixed that doesn't need fixing. All that happens is that you risk breaking something else.
Have fun.. but be safe with electricity.
