This has been addressed in the latest Sage Advice.
Thank you, @V2Blast
Sage Advice
Does the Thief’s Use Magic Device feature allow them to use spell scrolls?
Yes. The intent is that a Thief can use spell scrolls with Use Magic
Device.
DMG page 141 is also of some help here (Casting Spells):
The ability check is still required to cast from the scroll but, as a Rogue has no spell-casting ability score, their's is always treated as zero when casting spells from magic items. It would stand to reason that a check would be in order to cast the spell from a scroll as well, the only issue is that it is just a d20 roll for them.
If you don't have a spellcasting ability—perhaps you're a rogue with
the Use Magic Device feature—your spellcasting ability modifier is +0
for the item...
It does go on to say that your proficiency bonus (PB) does apply but this is specifically called out as an ability check so your PB does not apply to those. If it were a Wand or Staff, is what I believe the PB is intended to use.
To infuse an item, you need the nonmagical version of that item.
The artificer's Infuse Item feature states (TCoE p. 12, E:RftLW p. 57, WGtE p. 179; emphasis mine):
At 2nd level, you gain the ability to imbue mundane items with
certain magical infusions. [...] Whenever you finish a long rest, you
can touch a non-magical object and imbue it with one of your
artificer infusions, turning it into a magic item. An infusion works
on only certain kinds of objects, as specified in the infusion’s
description.
The Artificer Infusions section at the end of the class description states (TCoE p. 20, E:RftLW p. 61, WGtE p. 181; emphasis mine):
The description of each of the following infusions details the type
of item that can receive it, along with whether the resulting magic
item requires attunement.
And finally, the description of the Replicate Magic Item infusion states (TCoE p. 22, E:RftLW p. 63, WGtE p. 182; emphasis mine):
[...] Alternatively, you can choose the magic item from among the
common magic items in the game, not including potions or scrolls.
In the tables, an item’s entry tells you whether the item requires
attunement. See the item’s description in the Dungeon Master’s Guide
for more information about it, including the type of object required
for its making.
Each of these portions I've bolded make it clear that you need a nonmagical version of the item to infuse, whether you're using Replicate Magic Item or one of the other artificer infusions.
Even though the Dungeon Master's Guide doesn't actually say "[X] is the type of item needed to make [Y]" for any listed magic item (as far as I know), this sentence seems to refer to the fact that each magic item that can be replicated is essentially an enhanced version of a clearly identifiable mundane item.
For instance, boots of elvenkind are boots, so the clear implication is that you would need mundane boots to infuse if you want to use Replicable Magic Item to make boots of elvenkind. This follows the same logic as the Boots of the Winding Path infusion (TCoE p. 21, E:RftLW p. 62, WGtE p. 181), which does explicitly state:
Item: A pair of boots [...]
In the case of your example, the description of cast-off armor states (XGtE, p. 136; emphasis mine):
Armor (light, medium, or heavy), common
Thus, you need a nonmagical version of whatever kind of armor you want to turn into cast-off armor. If you want cast-off half plate armor, you need a nonmagical set of half plate armor to imbue with that infusion.
The Replicate Magic Item infusion doesn't create a magic item from thin air - it gives you the ability to imbue a certain mundane version of that item with the necessary magic to function as that magic item.
Best Answer
Artificers with the Magic Item Savant Feature only require Ability Checks to cast spell scrolls if it's a 5th level spell on the Artificer Spell List, and the Artificer is below level 17
Under normal conditions, a spellcaster is unable to cast (or even read!) a spell scroll bearing a spell that isn't on their spell list (Spell Scroll, Dungeon Master's Guide, pg. 200). However, as has been established, the Magic Item Savant feature allows them to ignore any class or level requirements to use a magic item, which includes spell scrolls.
Then, the requirement to make an Ability Check is worded like this:
Since Artificers gain the Magic Item Savant feature at level 14, the only spells that will ever fulfill this condition are 5th level Artificer spells. Any other spell is
Therefore, the only time an Artificer with the Magic Item Savant feature ever needs to make an Ability Check to cast a spell scroll is if they are between levels 14-16 and attempting to cast a Spell Scroll for a 5th level Artificer spell.
"But that seems... really specific. Especially since it also means they could cast 5th level spells from any other class' spell list without the check!"
Yes. At my table, I'd probably houserule it to simplify and just say that any Artificer with the Magic Item Savant feature no longer requires Ability Checks for any Spell Scrolls. The slight boost in power is worth, in my opinion, the time otherwise spent by players and DMs double-checking to determine if a 5th level spell is on the Artificer Spell List or not.