I have a suggestion you might want to think about.
Fix cover!!
Why, Just having the PLD in the party mitigates damage with abilities active. Since the PLD has the rule of taking the brunt of the damage (PROTECTING OTHERS) by not being a damage dealer then they should be able to mitigate damage by actively interfering with attacks. Creating enmity is only one half what PLD could be. Think about how other FF games that used cover. It would auto cover people who where attacked it didn't matter who was targeted. Right now the job is mostly trying to get the enemy to COME to them (warrior style) as an offensive tank instead of a defensive one.
The role of PLD should also be able to shield others by intercepting attacks sent to them. The cover ability is way to limited to be very useful. It's recast is to long, it's duration isn't log enough and it only protects one targeted person even if there is only one attacker. If this PLD person was protecting my family I'd feel better off protecting myself by running away. If someone wants me dead and they're set on killing me or someone in my family. A bodyguard trying to hold the attention of the attacker shouldn't be the only method of protection.
Priorities.
1st. Cover & protect
2nd. Redirect attention.
3rd. Disable or eliminate threat.
Instead of only: (You need more hate then others provoking the enemy somehow)
"COME TO ME MONSTER I'M MAKING YOU ANGRY!"
It should be: (I'll move to protect my allies. I'm going to COVER them with my life)
"TO GET TO THESE ALLIES YOU HAVE TO GO THROUGH ME FIRST!"
Think of it like this.
If there was an important group that a guard (lets say secret service bodyguard) was protecting a few high level officials. If someone started shooting at one of them they wouldn't just stand there yelling obscenities at the shooter. No they'd automatically run to cover that person with their body to PROTECT/SHIELD them. This would redirect the attackers attention to getting past the bodyguards cover attempts. Last the bodyguard would attempt to eliminate the threat while keeping the targets safe or giving them time to move to safety.
Currently for the most part when in FFXI combat the role of tank is to YELL at the target or hurt it enough to redirect the attacks towards the PLD tank. The tank should also have a high level of skill to protect allies (to a degree not perfect) REGARDLESS of who the enemy is angry at by intercepting its attacks. PLDs by the games design are stupid. The job is mostly designed hold the attention of the attacker. It should also mitigate damage to others by taking a more aggressive role in actively covering them and making them feel protected.
On a side note:
If the developers reduced the recast to 30 seconds for a single target that last 15-20 seconds. They could make cover based tanking an advanced gaming skill. This would allow the PLD to protect someone every time the ability is used but still require holding hate to an extent. If modified this way the job would be changed in ways where it would be a more useful tank but the player would still need skills to preform well. This would definitely change the tanking tactics for this job.
Enmity would still need fixed and the issue you stated with instant attention updates would still be a a problem but, at least the job would be more likely to get a slot for tanking. Also with covers the tank would have no evasion and counter capabilities. Shadows and flash blindness could be an exceptions because technically they are creating fake clones or blinding the enemy.
Also, Since cover is a sub job ability it would be capable of making other jobs more effective at tanking when using the modified cover. They still wouldn't be as defensive or successful though as native PLD.