I agree with 1 and 4 more or less.
1: PLD is seen as the selfless knight, who protects others by standing in front of them. Therefore he doesn't get the shield, only the others. Playwise, it would be nice to just pop it and everyone including the PLD would get the shield. But then again, the healing via holy spirit triggers it as well if I recall it correctly and with PLD being a tank who get's less damage anyways, it's not as needed on himself then on others (healers and DPS). If I had to choose, I would remove the healing requirement rather than putting the shield on the PLD as well. Would be a bit more casualfriendly.

2: we had that in Stormsblood actually. My Co-Tank used Cover + HG on our WHM in O4S to soak the Aero TB. We will probably never go back to that because it's overpowered. cover had 20% damage reduction as well and I think it wasn't bound to the gauge then. Personally, I would at least give cover some love by giving it a damage reduction. Like blocking the attacks 100% or so.

3: Circle Mitigation? No. Reason: it wouldn't make any sense. The whole thing is meant to be a cone because the PLD is getting hit frontal and blocking it with it's shield, creating a savezone behind him. Also: the skill is in my opinion mostly only useful in Boss encounters like (ex) Trials or Raids, where people usually stack up so that healers can heal up quickly. Standing in front of the party and use passage isn't a problem. The cone is also wide enough so that it shouldn't be a problem to get everyone in. The damage reduction itself should be fine (15% less damage), because similar to divine veil: you can use it between 2 GCDs, while everyone else get the buff for around 5 seconds which is more than enough to mitiage raidwide damage (except they changed it again and I don't know it, because I haven't used it in Endwalker yet). The only part missing then is the 100% blockrate for the PLD himself

4: it would make shieldbash see some usage again since PLD has lowblow as stun. I would prefer a 30 sec duration, so that it lines up with circle of scorn and expiration (or whatever the upgraded Spirits within is called again). Then again, we would have more oGCD to weave then.

5: if I understand it correctly, you want to make the Blade combo to an PLD Continuation? the oGCD spamming ability of GNB? I mean it's fun to use when playing casually GNB but... I wouldn't say it's a good thing for PLD. With Shieldbash, PLD should have 3 oGCDs landing in the Requiescat window, which should already ease a bit of the 1 button spamming. But I do see your point, that the holy spirit spamming can be a bit boring. About the weaponskill change... I would rather grab an idea from another thread: making Skillspeed and spellspeed into 1 stat, because that issue also applies to other Jobs (looking at you Dark Knight) as well