Let me break it down for you one at a time...
It isn't +50 damage to auto attacks. It's a second hit on your auto attacks worth a potency of 50. This means you essentially get a permanent DOT that hits more often than a DOT because out AA is actually faster than you think on Paladins. Further, this is augmented by things like Strength, Attack Power, etc. So it actually results in a higher damage increase than just 15%
Refine your rotation. Flash applies a 10 second Blind on the first application. The second time you Flash it will apply 5 seconds of Blind. However, what you fail to realize about debuffs is that if a debuff that is weaker or is trying to override one that has more duration than what will be applied is attempting to overwrite the current one, it won't. So the second flash won't apply unless the current application has less than 5 seconds to go. This means that in the first 5 seconds, you can flash all you want, but it won't override the current application. After that 5 seconds, avoid flashing, or you'll clip your duration. My rotation goes like this Flash, Fast Blade, Flash, Riot Blade, Flash, Goring Blade, switch target, Fast Blade, Flash, etc. What you see here is that the first flash applies 10 seconds of blind. The second one won't apply because the current blind has more than 5 second to go still. The third flash will apply, but there's only about 1 second left on the current blind. This third one applies about 5 seconds of blind. The fourth one will apply about 2 seconds of blind, and will apply just about the same time that the current blind falls off. Circle of Scorn is also generally used between that first Goring Blade and the second Fast Blade to bridge the enmity gap between them for more consistent enmity.
Everyone wants to block magical damage. And I admit, that would be nice, and it wouldn't be over powered either. I can see the frustration here, but at the same time, you kind of need to suck it up. And don't scoff at MP regeneration. The amount is equivalent to what Flash costs, and about 1/3 of the cost of Clemency, which allows you to actually use Clemency about 3 times before you go OOM.
This skill is actually our highest potency attack. You underestimate the actually damage that comes from a DOT. This is augmented again by your Strength and Attack Power. So it's like having a slightly weaker version of your Shield Oath up for 24 seconds. It's also changed by Skill Speed as well. DOTs received a big boost in strength with 3.0 from Skill Speed.
If timed properly, this skill is actually quite easy to use, and it is augmented by your Strength as well. I have had crits of 6-7k from this skill, and the full power of it comes from casting it on another player. Thanks to cast clipping, you can actually get this heal off in less than 3 seconds. This is displayed by mages who cast clip as well. They usually start moving at about the last .5 seconds of the cast, and the spell still triggers, despite them moving. Also, when this skill is used on another player, you also get half the healing that player got. So if you crit for 6 or 7k, you'll see healing for about 3-3.5k, for a total healing of about 9-10.5k. No other casted heal can boast such number in this game without using some kind of augmentation cooldown, not even Cure 2. This literally has a potency of 1800 if cast on another player.
Rage of Halone applies a STR debuff that you should ALWAYS have up, so even as OT, you should be using RoH just to keep up that debuff. Royal Authority is great, but again, fights change based on the damage coming in. Thanks to Shield Swipe procing off of physical damage due to block, battles with physical auto attacks see more Shield Swipe procs, and thus is should be used more often to help maintain TP. For this reason, it is true that Royal Authority is rarely used as MT. However, take a fight with mostly magical damage. A4 comes to mind. You'll never see a proc of Shield Swipe, so keeping up the RoH debuff and the DOT from Goring Blade still leaves you with enough time before having to reapply them to also do Royal Authority. So change your rotations based on your fights. Physical fights, I only use RoH and Goring to keep up both debuffs while also using Shield Swipe whenever possible to manage TP and increase Enmity. Magical Fights, I cycle through all three rotations to keep up debuffs as well as do more damage.
This ^ and remove the TP requirement and the Pacification from Shield Swipe and give it the same potency as RoH are my only two wanted changes.
I've used Clemency specifically when a DPS is dead and the healers are down on Swiftcast and I am OT. Or when in A2, I am tanking the last spider, and my OT is being beaten on by the group of like 5 adds with the two Jagd Dolls. I cast Clemency whenever possible to help handle the spike damage better. Also, if Sheltron blocked all physical attacks for 10 seconds, its CD would have to be much more massive, and I see it being too much of a copy of things like Bulwark and Raw Intuition, as Bulwark already almost guarantees this with how strong it is. I think Sheltron is quite fine where it is, good for casual use, but versatile for big tank crunches.