I think FoF should just upgrade into Requiescat, and make Requiescat have range so it has the full functionality FoF does. I hadn't thought of Holy Circle being combined with Holy Spirit as a cleave target on an enemy, but that honestly makes a ton of sense. Goring should just be removed with its potency folded into Confetior or some of the sword attacks in the combo. Shelltron/Intervention could get the Heart of Corundum treatment. Just remove the CD, it's limited by Oath Gauge so you can't just sit there and machine gun it. You can only use up to two at a time, and it's not ridiculously overpowered in a game where Nascent can generate basically the same situation by effecting both a target and the WAR (the WAR doesn't get the mit but gets the healing, which is the lion's share of the functionality of the ability). This wouldn't really be OP, not to mention PLD already pays in lowest damage of the Tanks, so having higher defense/party utility is supposedly the exchange for that. Spirits Within/Expacion and Circle of Scorn are still kind of "why are these even here?" buttons. APM, I guess, but just give us more charges/faster recharge on Intervene (gap closer) so we can smack that more in burst.
That's six hotbar spots right there. I kind of agree with you that it's annoying to have so many buttons that are "just use once every 1/2 mins or so", but it's even worse when its abilities that you always use together anyway. Abilities that, if the macro system didn't suck, you could just all macro to a single button because they literally don't do anything else (the only reason this doesn't work is because macros not quing; otherwise the buttons bring that little to the table).
Shield Bash actually IS useful in solo content, Deep Dungeons, and Exploration Content. I think a better idea would be to give the button a context upgrade of some kind. For example, as I've pitched, if we DIDN'T remove Goring, have it be the Shield Bash button "context upgrades" to Goring Blade when you use Requiescat, like how RDM's Thunder/Aero/Jolt upgrade into Flare/Holy/Scorch/Resolution, or how Confetior changes to the three swords abilities one after another. Gets rid of Goring hogging a hotbar spot while keeping Shield Bash's general functionality (Goring can even have the Stun, doesn't hurt anything since bosses are immune to it anyways).
Cover is a really useful ability for saving runs or pushing prog, the issue is the gauge cost. There's no reason for it when it already has its CD. Just remove the gauge cost and Cover would be a useful ability again.
Clemency is honestly pretty good where it's at, and the one Tank healing button I think IS in a good place. Yes, it's powerful, but a total DPS loss to use and will make you go MP negative really fast. It's useful for pushing prog or trying to salvage fights, but isn't outright replacing Healers (PLD doing its damage rotation and letting the Healer heal is always going to be a party DPS gain in stuff like 4 mans over the PLD trying to be the healer and letting the WHM Glare more). That trade-off is what makes it good and makes things like Nascent or Shake bad.