The PC Gets a Save
There is no effect line in 5e's magic spells, so when it says affects another creature its not calling to a mechanical keyword, but saying it shifts target. Spell Reflection is basically used to turn the party's spells against them, but a targeted PC should still get to make the normal save as if he/she was the original target of the spell.
No, the Ring of Spell Turning does not work on spells that target more than one creature for any reason
you have advantage on saving throw against any spell that targets only you (not in an area of effect)
So the ring gives you advantage on saves from spells that only target you but that is not an area of effect.
A typical spell requires you to pick one or more targets to be affected by the spell's magic. A spell's description tells you whether the spell targets creatures, objects, or a point of origin for an area of effect (described below).
Many spells can target multiple creatures in their effects and these are not considered to be targeting an area.
If a spell like hold person, which targets creatures and not an area, is upcast such that it allows an additional target then that spell is now targeting more than one person. Thus, the Ring of Spell Turning will not grant advantage against it.
When you cast this spell using a spell slot of 3rd level or higher, you can target one additional humanoid for each slot level above 2nd
Jeremy Crawford has confirmed that up casting a spell would indeed increase the number of creatures targeted by that spell:
Can a spell at its current level target more than one creature? If yes, you can't twin it.
This is in reference to the twinned spell restriction that says:
When you Cast a Spell that Targets only one creature and doesn’t have a range of self...
Crawford says that upcast spells that target more than one person cannot be twinned. That means that spells that are upcast that target more than one person do indeed count as one spell that is targeting more than one creature.
Going back to the question at hand, this means that an upcast spell that targets more than one creature definitely will NOT be a spell on which the ring will give you advantage.
Regarding a twinned spell:
...you can spend a number of sorcery points equal to the spell’s level to target a second creature in range...
The untwinned or not upcast versions of the spell may only target one creature, but upcasting or twinning them causes that spell to target more than one creature.
Thus, regardless of the reason, if the spell does not target only, then it is not affected by the ring. This is the case with hold person in both upcast and twinned varieties.
Best Answer
It doesn't reflect fireball.
The key words are "The spell targets the chosen creature instead of the Morkoth."
"Do B instead of A" means "In a situation where you would do A, don't do A, and rather do B." (You are doing B in A's "stead".)
Fireball doesn't target the Morkoth, so it can't target another creature instead of the Morkoth.
"But fireball says 'a target takes 8d6 damage'"...
Yes, but it's wrong. Those aren't targets. These are targets:
Note that the caster picks "one or more targets". For fireball (and area-damage spells generally), you don't pick the specific creatures; you pick the point of origin.
The rules sometimes, confusingly, use the word "target" to mean "anything affected by the spell". For example:
Why do I acknowledge the point of origin as the real target of the spell, rather than the "targets" who are making saving throws? First, because one of them is described in a paragraph titled "Targets" in large bold letters. And second, because of this:
That seems fairly conclusive: for purposes of the spellcasting rules, the target of fireball is the point of origin. Since you can't choose the Morkoth as the target (you have to choose a point!), its Spell Reflection can't choose something else as the target "instead".