GOOD SPEL

Most DMs with no notes on pc acs ask "Does x hit?", tbf

What happens when it's up cast?

Until the start of your next turn, you have a +5 bonus to AC, including against the triggering attack

Nothing extra. You just used a higher spell slot.

SOME GMs might object to it, since it's not explicitly listed with upcast benefits, but I think most are okay with it. 

you can always cast a spell with a higher spellslot regardless if the spell can be up cast or not  

Why don't your consider the reaction to he part of your turn? Reactions can happen outside of your turn, but in this scenario it is on your turn

 You can trigger an attack of opportunity without ending your turn.

this should be once a long rest

I'm sorry but I believe you're wrong on the use of manifest. appear is the appropriate noun for this example but I do understand that manifest sounds cooler.

Assuming you mean spell scaling it does nothing to the spell you just use a higher lvl slot unless it's added in the description that the spell can scale it won't plus I don't think a dm would even debate you on that.

That seems excessive considering the multitudes of ways to increase ac without this spell especially if your dm is running with magic canceling barriers, items, and counterspell. On paper it looks broken i understand but who is gonna actually take the time to try and abuse a most likely single instance spell that doesn't last. Unless someone builds specifically to not get hit with other abilities boosting ac but I mean they ain't gonna do anything else at that point and that's a reaction.

Could be wrong about the exact scroll rules but as far as I know there is no rule you have to scribe on paper or similar.

If it is on your classes spell list AND are proficient in arcana AND another craft (woodcarving/smithing/...) you could scribe the shield on many objects.

Came across your comment looking up some details to pitch my DM turning my maul into a "wand" of shield. My radiant soul always makes sure the bard has at least one Dagger with healing word scribed on it. Keep in mind scrolls are usually consumed. The DM allows me to reuse ("charge:) the weapons but that is up to the DM.

As an artificer armourer tank. Would this be worth trading +1AC to use this once per short rest? I'm thinking of asking my DM for a shield amulet( 1 use of shield per short/long rest) infusion and trading out a +1 armour infusion. Would take me from 20 to 19 AC. I'd say we have about 2 battles per short rest. Just went to level 7. Cheers

If being hit (successfully) triggers the reaction, and that results in the hit not becoming a hit b/c of the AC boost (which applies to the triggering attack), you didn't get hit and therefore the reaction wasn't triggered.  Paradox.

A-Tier

Inexpensive, Easy to get at the very start, Great effect that can make the difference between life and death.
Easily one of the best and iconic spells in the game. Would never go anywhere without it personally, regardless of the build.

Quote from AnihcNiedam >>

No, they don't. Tal'Dorie Campaign Setting is not an official source. It is published by Green Ronin Publishing, not Wizards of the Coast, and so it is 3rd party material.

With that being said, I don't think their ability is too powerful or anything. It also wouldn't matter here anyways as Shield lasts until the beginning of your next turn, and so having another reaction wouldn't do anything. Spells with the same effects also don't stack like that.

 Mind of mercury. Critical Role way of the cobalt soul, check it out

IMO, It's better to have more mechanics and less flavor text. Flavor text is the cause of probably 80% of rules arguments as people try to turn flavor into rules. I greatly prefer the rules to just be rules and let the players and GM add the flavor as appropriate to the campaign setting.

Yeah nope, I don't wanna get hit right now. (My fighter who took magic initiate wizard)

its up to the DM then

it also makes the hitting attack have ti react retroactively to its ac i thought? but maybe thats another one you make Home rules for