The error message could indeed be the floppy drive cable not connected properly, however I've not diagnosed this kind of problem for a while
Make sure to line up the red stripe/dashed wire on the floppy drive data ribbon cable with the pin 1 marking on the A500 motherboard! You can unplug this and reconnect to make sure, also check for bent and/or broken pins here, it can be easy to do if you're aren't careful.
As for the trap door memory, yes CBM used to shield the ram and clock with the sheet metal can you see, it's kind of wedge shaped to fit into this slot properly! If memory serves me, there are some smal metal tabs that are bent over, use some pliers to straighten them out and see if the sheet metal will come apart, there could also be a solder joint or two around this sheet metal to keep it in place and give a good ground to the shielding!!!