A500 Boot woes

[Boing-ball]

1st off, welcome to Amiga.org. Word of warning, this place is full of Sheldon’s who might want to start a 2-bits compliment argument with you 😆🤣👍🏻

Could be the Denise chip for the Video fault. Problem will be, even if you get a DiagROM chip, if you are not seeing a display then you will need a Serial connection to another computer such as PC to see any output.

I doubt it’s the Kickstart ROM as you have identified activity with the floppy drive. But you never know when it comes to Amigas.

Check out Stu’s videos (SHG Retro) on Amiga A500 and A500+ repair here:

https://youtu.be/CxARHzGi6Vk?si=qWtMGYgy5z_tsdz7

Also Chris Edwards channel:

https://youtu.be/J3HXOGkVCXE?si=DbFVziptmSs71t2M

[Boing-ball]

Without some sort of working input device, then ideally you need to get into the DiagROM menu to proceed further.
As for the mouse. Only 1 port can be used for the mouse. The other is dedicated to Joystick/Joypad. The mouse port can also be used for a second joystick.
You can try swapping the CIA chips over to see if that helps with the mouse input. Fingers crossed not both CIAs are at fault.
But from the looks of things you have a few issues going on. CPU and Chip RAM are not an issue from the looks of things.
There are also micro fuses/resistors on the board that might have popped for the mouse to also not work.

CIA chips can be bought second hand plus other A500 bits and bobs:

https://www.ebay.co.uk/itm/387439406742?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=7irsl2-xrhg&sssrc=4429486&ssuid=udhysnkvtsy&var=&widget_ver=artemis&media=COPY