Straight from the DMG Errata:
Attunement (p. 136)
The first paragraph ends with a new sentence:
“If the prerequisite is to be a spellcaster, a creature qualifies
if it can cast at least one spell using its traits or features, not
using a magic item or the like.”
emphasis added
Therefore, for the purpose of attunement:
If a creature is able to cast a spell by virtue of their own ability (not from an item), whether that be race, class, or feature, you are a spellcaster.
Ideally, Sacred Flame kills it (roughly) twice as fast
A simple formula answers this, with X being the number of rounds needed to kill the Shadow.
1d10 averages 5.5, halved is 2.75
1d8 averages 4.5, doubled is 9
For Firebolt, you hit on 8-20 (chance for crit), so .65 x (5.5/2)(crit bonus)x(XFB) >= 16
(Need to hit AC 12; roll 1d20 +2 Int +2 proficiency)
(14 / 13 = crit factor; (x+1)/x for damage with the chance of crit thrown in)
For Sacred Flame, you hit on 13-20 (and no crit) .4 x (4.5 x 2)(XSF) >= 16
With a DC of 11 (you only have +1 due to your 12 Wisdom)
(The Shadow makes a dex saving throw versus the spell)
Solve for XFB: 16/(.65 x 2.75 x 1.077) = 8.31 rounds
Solve for XSF: 16/(.4 x 9) = 4.44 rounds
As a practical matter, being able to kill it in ~4 rounds rather than ~8 rounds means that your Sorcerer with that meager 16 HP has a far better chance to survive the fight before having no offensive capability.
Actions
Strength Drain. Melee Weapon Attack: +4 to hit, reach 5 ft., one creature.
Hit: 9 (2d6 + 2) necrotic damage.
Two hits and your Sorcerer is down.
This is an encounter were the Cleric's "turn undead" would be a handy skill to have.
This whole problem also presumes that you stay more than five feet away from the Shadow. In the case of firebolt, since that is a ranged spell attack rather than a spell that calls for a save, if the Shadow closes to melee range your firebolt attacks at disadvantage.
(SRD p. 103; spell attacks) Most spells that require attack rolls involve ranged attacks. Remember that you have disadvantage on a ranged attack roll if you are within 5 feet of a hostile creature that can see you and that isn’t incapacitated.
Best Answer
Per the DMG, no, the Magic Initiate feat by itself does not enable one to create magic items.
Page 128 of the DMG describes the requirements for crafting magic items, and specifically states
Although the Magic Initiate feat (arguably) does turn a non-spellcasting character into a spellcaster, it does not give a character spell slots; it only gives a character cantrips and the ability to cast a single non-cantrip spell once per long rest.
Per XGtE, even characters with no spellcasting ability whatsoever -- not even spells from Magic Initiate -- can craft most magic items if they have proficiency in Arcana or the relevant tool.
The alternate, and wholly optional, system for crafting magic items presented in XGtE, at p. 128, permits any character to craft a magic item, not just spellcasters. (Whether they have the Magic Initiate feat remains irrelevant.) However, the character does need some applicable proficiency:
There are two notable exceptions. First, brewing a potion of healing specifically requires proficiency with herbalism kit. Second, for spell scrolls, the crafter must indeed be a spellcaster. See XGtE p. 133.