1. This only hold true in a set static raid scenario, and there you also have to wait for DNC and/or AST (also BLM, SCH/SGE shield prep etc.) for at least 16s. This is not an issue, and it only adds difficulty in programming and ability effect like *outside of combat* stuff. We already have actions with wall of text that half the community can't even be bothered to read, let alone understand it (yeah, I know that's not really a reason, but still...) don't make everything unnecessary complicated.

2. I can get behind that. I would even argue PLD should be able to trigger it themselves, or it auto-triggers after the duration expires, similar to AST star. For anyone saying that PLD has good mitigation with passive block or strong healing on every rotation repeat: other tanks have more healing options even before 84, AND at least 1 additional CD. Also block doesn't work on crits, which can render Sheltron useless (also stun/down for the count debuff). Getting their own Viel shield is hardly a significant buff. (Shield on dps/heal is roughly the same as Shake, ut weaker on tanks, so there is that, too.)

3. You completely break PLD rotation by doing that. You might not want to use 2 Atonements outside FoF window, but during it you wanna use all 3! PLD has the most flexible dps tools to optimise your rotation and dmg output. By cutting their Sword oath stacks, you also cut their options. There is also the point before getting Atonement; you still use 2 full Royal Authority combos, so you can't change Goring blade too much or you risk breaking PLD on every level! Leaving 1 stack of Sword oath unused is unique, while being a gain on most occasion, just leave it be.
On another note - as aodhan said; by reducing spell GCD recast with SkS/trait, and leaving out 1 Atonement, PLD would come closer to a 60s rotation. Adjust spell recast with SkS on physical jobs (PLD/DRK).

4. I kinda agree with you here, but you are selling Hallowed Ground way too low. In contrast to Walking dead, HG shields you from damage which allows you to cheese or outright ignore certain mechanics. WD cannot do that! Same with Superbolide. I agree it should have a lower CD, as Holmgang still remains supreme even after several recast nerfs (some at SE headquaters apparently thinks putting tank busters timings 4-5 min from each other is just right, so only WAR can cover them)
I suggest a CD reduction of WD to 4:30, SB to 5:00 and HG to 5:30. This way, DRK can cover some tank busters that only WAR is currently capable of, while GNB and PLD might see more invul usages than just "once in a life time".