If you bothered to read. you would have noticed the word "everything."
But if you want me to break it down, I can.
Yes, it is nicer and generally more productive to talk about differences of opinions rather then just "you are wrong". Thank you for the discussion!

-The choice/direction to give such abilities to roles outside of healer encroaches on the role.
This one is strange. This is like saying Scholar shouldn't have damage mitigation because it encroaches on Tank roles. Or White Mage shouldn't have any DPS capabilities. It's not and has never been that type of game.
I believe this stems from the fact that you can keep a party healthy with liberal use of abilities from various jobs without healer interaction. While I am more then ok with other jobs getting heals/mits/abilities I do think that it breaks down the trinity to allow for jobs to do what a healer does fully.

-Every job in FFXIV has a 'filler GCD' skill that they press when there are no other requirements. No role in the game pushes this skill more than healer. Not by a long shot. Our offensive capability should still feel dynamic and rewarding. Yes, we are healers, but we are also casters.
Literally contradicts the first point.
I can get how you would arrive at that conclusion based on your first point, but again more then ok with other jobs having abilities. What I want is to not fall asleep between Ogcd buttons from spamming 1. And no, I press more Ogcd's as a summoner then I do a healer because of the nature of that job, so examples can be pulled from anywhere for that. Really, this stems from the fact that we press 2 buttons, with a 3rd every 60 seconds now, when healing on a large part. I have even read posts where people have to replace, you guessed it, the number 1 key on their keyboard from pressing too many times while healing.

-Barrier/Pure healer split is largely redundant. All healers essentially play the same with the exception of few and far between niche abilities.
Contextual. All Tanks play the same way, just like all Mag and Phys DPS play the same way.
I mean, they do and they don't. Barrier healers need to pre prep for large party mechanics while pure healers mostly fix the aftermath. Just a quick and dirty example but the gameplay difference is there. With this being said, I do agree with you on the larger part though. Jobs feel kind of the same with eachother and tbh I wish they had more individuality but I still have fun with it

-These are not only abundant, but exceptionally powerful. This in addition to self-sustain on other roles reduces the dynamic of healing comrades.
An outright lie. A reduction from what? Zero?
I think I am confused here if I am being honest. There is some strong self-sustain out there. I mean, I have warrior friends that actively tell me not to heal them during wall2wall pulls. With that being said, I don't want jobs like tanks to lose their cool abilities because that is literally their job is to tank hits so I feel like this kind of discussion helps for sure.

-Healers should feel like they have a place in all forms of content. It is understandable that difficulty can't be so great as to bar progression of story-based content. However, innovative means should still be employed to make greater/full use of abilities.
Player error.
Go run dungeons without a tank and then tell me player error. This is an MMORPG, lets be a party. If you wanted a single player game, then please check them out cause there are some cool ones that let you feel like the lone wolf hero.