Death by many Magic Missiles
Let's start with when to make Death Saving Throws (PHB, 197)
If you take any damage while you have 0 hit points, you suffer a death saving throw failure.
So any time something gives you damage while at 0 hit points, you suffer a saving throw failure.
As quoted above in the question, the language for Magic Missile is separate per missile. Each missile strikes simultaneously, but the damage from each is separate. If we are separating damage, we are separating those Saving Throw failures.
The Concentration function is similar in that it triggers off of damage (PHB, 203):
Whenever you take damage while you are concentrating on a spell, you must make a Constitution saving throw.
However, it also goes on to state:
If you take damage from multiple sources, such as an arrow and a dragon’s breath, you make a separate saving throw for each source of damage.
Jeremy Crawford specifically ruled on Concentration saves and Magic Missile requiring each missile to force a new save.
Roll for each missile
The real question
That leaves us the real question here as to whether or not Magic Missiles are separate sources of damage, or if the missiles hitting concurrently represent a single taking of damage.
Given that Crawford seems to believe that magic missiles are multiple sources that require a roll per source, it seems you can extrapolate that the missiles are also giving damage separately (while hitting concurrently) and thus forcing death save failures for each magic missile. The concurrent nature of the strike doesn't override the multiple deliveries of damage from separate sources.
Caveat Emptor
If you are a DM and planning to target a PC like this, you must beware of hard feelings at the table. Typically death saves are a PC thing, and if you are going to remove that by going after unconscious players then the possibility of hurt feelings is real. Most monsters are more concerned about taking out other creatures who are alive rather than double-tapping any downed creatures. If there was a real reason by the NPC to do this, then that's different - but if you're just going after unconscious players because they're easy targets to completely kill you may suffer backlash from your table.
No, spells from magic items aren't upcast unless they specifically state it. From the DMG, page 141:
Some magic items allow the user to cast a spell from
the item. The spell is cast at the lowest possible spell
level, doesn't expend any of the user's spell slots, and
requires no components, unless the item's description
says otherwise.
In particular, the description of spell scrolls does not say otherwise.
Best Answer
Yes and No. Rings of Spell-Storing and the Artificer's Spell-Storing Items are different.
The Spell-Storing Ring is a specific magic item, whose text says the following:
Since the spells are cast to store them in the ring, it would be possible to cast these spells using higher-level spell slots. When you do so, the spell would be treated as a spell of the same level of the spell slot expended; if you cast Magic Missile into the ring with a second level spell slot, it would be a second level spell, take up two levels of space within the ring, and produce an effect as though it was cast with a second-level spell slot.
However, by contrast, an Artificer's Spell-Storing Item, despite having a similar name, works entirely differently:
No spell slots are expended; when you produce a Spell-Storing Item that contains Magic Missile, you can't expend a second-level slot to make it fire an additional missile - and Magic Missile is only a second-level spell when cast from a second-level spell slot.