1 - Those screws were made for the terminal strip manufacturer and probably are too special to locate separately. “Borrow” some from other terminal strips on the panel you are not using. To get replacements you would probably have to find a terminal strip to buy and use those screws.
When I was working and ran into that we would look for a board going to repair or scrap and get what we needed from there.
2 - That is a problem in that detector. It could be that it never worked and the detector was replaced. Sometimes the LED is not soldered in but plugs in. (I don’t know System Sensor detectors.) If someone opened the detector for cleaning and knocked the LED out it may have been put back in backwards or a lead was broken.
Honestly, I got the detectors new. The LED does actually blink every so often but doesn’t durning alarm. Also, I’ve got resistors in zone terminals +1, -1, so on all the way to -8. I’ve also got them in S1A+, S1A-’ so on until S2A-. Everything still goes into trouble, other than a few above the main controls. Although, some led’s may be broken. Also, manual evac won’t work.