2 - You cannot really fix this unless an alternative F-Droid client is installed as a system app by the manufacturer, or they allow relocking the bootloader. Good luck convincing them.
5 - I can run anything of any age on my devices, accepting the security risk. I want to be able to factory reset and use one of my Android 4.4 phones with an unmatched speaker as an Internet radio receiver instead of throwing it out. F-Droid explicitly tells you how long it’s been since the last update and ranks old apps low in lists and searches.