Amottica's explanation covers most of it, although ZOS has said they have tried to fix the stuck-in-combat bug before. It still persists, unfortunately.

I'm curious about #5. What is suggesting to you in the moment that a player's mag or stam is "remaining at 100% at all times"? The game does not show you those stats for anyone except yourself. Logs can show it, but even live logs require some processing time. (If you see players staying at 100% magicka and stamina throughout an encounter while they're casting abilities with costs, I'd ask both the people at ESOLogs and ZOS to look into what's going on there to ensure it isn't a bug with the logs.) And why do you expect to see magicka or stamina drained when damage is taken? Unless you're referring to when someone takes damage while blocking, which will only drain the respective offensive resource if the attack is blockable in the first place. The taking of "huge damage" might suggest the attack was unblocked, as blocking provides at least 50% mitigation against things that can be blocked. Many DoTs are unblockable, though, as are even certain direct attacks, like Power of the Light - the blocking resource will not be drained if the target is blocking when that is applied or explodes.