As written, there is no need Spare the Dying succeeding for the subsequent healing.
The Revival Protocol feature, in this version, does two things in this precise order:
- It allows the drone to cast Spare the dying
- It restores 1d4 HPs.
The second step has no requirement for the spell to be successful, the word "then" in the description refers to the order in terms of timing. Note that if the target is an invalid one for the spell, then nothing happens1, except for the spending of the slot (or the number of uses for features like this).
Moreover, the healing part of this feature seems to work also on constructs: indeed, official materials provides with features that heal creatures but do not work on constructs (and undead). For example, see the paladin's Lay on Hands:
Your blessed touch can heal wounds.
[...]
This feature has no effect on undead and constructs.
So, Spare the Dying does not work on constructs (and undead) as usual, but the healing works just fine.
As written, casting Spare the Dying has an impact only on particular cases.
Since the second part of the feature does not require the spell to succeed, restoring 1d4 HPs already saves a creature:
The best way to save a creature with 0 hit points is to heal it.
If you heal a creature that is at 0 HPs, then they are no more unconscious and they do not have to make saving throws.
There may be some situations in which healing has no effects, such as on targets of Chill Touch cantrip: in this case, the stabilization from Spare the Dying plays role for a creature with 0 HPs, but the healing does not work2.
As written, this feature can be used to restore 1d4 HPs to a wounded creature, and also to revitalize a dying creature, but Spare the Dying has no role at all.
1 Credits to Joakim that pointed it out in the comments.
2 Credits to Molot that pointed it out in the comments.