Yes, advantage would negate disadvantage for purposes of Sneak Attack.

According to the rules on advantage and disadvantage (PHB 173, emphasis mine):

If circumstances cause a roll to have both advantage and disadvantage, you are considered to have neither of them, and you roll one d20. This is true even if multiple circumstances impose disadvantage and only one grants advantage or vice versa. In such a situation, you have neither advantage nor disadvantage.

So, for example, if you (the rogue) have both advantage and disadvantage on an attack roll against an enemy while your ally is within 5 feet of the target, you could make a Sneak Attack because you are treated as if you don't have disadvantage when making the roll. This complies with the requirements for Sneak Attack (PHB 96, emphasis mine):

Once per turn, you can deal [extra] damage to one creature you hit with an attack if you have advantage on the attack roll.... You don't need advantage on the attack roll if another enemy of the target is within 5 feet of it, that enemy isn't incapacitated, and you don't have disadvantage on the attack roll.

However, if the ally were absent in that situation, you could not make a Sneak Attack because you are treated as if you don't have advantage when making the roll.

The same rationale would apply for features other than Sneak Attack that depend on either advantage or disadvantage on any attack roll, ability check, or saving throw. When any such roll is made with both advantage and disadvantage, the roll is made as if it actually has neither advantage nor disadvantage. In this case, no feature which requires advantage or disadvantage would be activated, and no feature which forbids advantage or disadvantage would be precluded.

The attacking enemy has disadvantage on the roll.

The relevant quote from the PHB is here:

If circumstances cause a roll to have both advantage and disadvantage, you are considered to have neither of them, and you roll one d20... In such a situation, you have neither advantage nor disadvantage.

It's pretty clear that, in order for the advantage and disadvantage to cancel out, the roll has to have both advantage and disadvantage. To use computing terms, it's an exclusive OR gate. A roll can have advantage or disadvantage, but if it has both, it gets neither.

The Elusive class feature could have been worded in a number of different ways, but the way it is worded makes it clear that a roll can never have advantage against a non-incapacitated Rogue 18. The only way that a roll can have advantage and disadvantage cancel out is if it has both at the same time. Since a roll can never have advantage, it can't have both, and instead keeps the disadvantage.