The rule on Unseen Attackers and Targets reads:
When you attack a target that you can't see, you have disadvantage on the attack roll.
So this part of Darkness works as you believe. Note, without the Devil's Sight it would be an even roll because they would have advantage attacking a blinded creature.
However, the rule on spell targeting reads:
To target something, you must have a clear path to it, so it can't be behind total cover. If you place an area of effect at a point that you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.
So, you aren't correct in assuming you can't be targeted by a spell, as darkness isn't an obstruction, only a visual issue. Spells that say "that you can see" would be stopped by darkness. So, you'd still be able to be the target of save spells. If this weren't the case, blind spellcasters couldn't exist.
The mirror image spell says:
Each time a creature targets you with an attack during the spell's duration, roll a d20 to determine whether the attack instead targets one of your duplicates.
But it also adds:
A creature is unaffected by this spell if it can't see, if it relies on senses other than sight, such as blindsight, or if it can perceive illusions as false, as with truesight.
The phrase "if it can't see" can be read either "not able to see anything" or "not able to see the target". In this case, the rules are ambiguous, but the lead Game Designer, Jeremy Crawford, tweeted:
An attacker isn't affected by mirror image if it can't see the images. #DnD
At the time, his posts were considered official rulings. Since then, WotC has made it so that only rulings published in the Sage Advice Compendium are considered official. So, it is the designers' intent (Rules as Intended) that darkness or invisibility would make mirror image's effect void. Though as Rules as Written, it seems it might work, if you have a lenient DM.