You can get into the back of the mule custom if it doesn't have the Reinforced Riot Armor mod installed. This is an oversight/design choice by the original creators at Rockstar.
If you'd like to have this changed, please create a suggestion
Rejected
Animals despawning upon death is a FiveM networking quirk and not something we can fix. As for them requiring multiple hits to go down, this is unfortunately also a combination of the aforementioned and the underlying GTA system/annoyance of NPCs tanking rounds like their skin is made out of...
Unable to reproduce and as there have been no other reports, we cannot look into this. My only suggestions would be to(should you still be experiencing this issue to this day) verify both screen resolution/aspect ratio or to toggle "Fixed-size NUI" on in the core FiveM game settings.
Rejected
Excellent news! This has now been fixed by the development team and will be released in a future server update.
Thanks for doing your bit to help us keep the servers bug free!