Peyton throws a WR screen to Harrison. Stokely, Wayne, and Clark get in front of Harrison, with only safety Roy Williams between Harrison and the end zone and needing to be blocked. Suddenly TO, Terry Glenn, and Julius Jones run off the sidelines and tackle the three Colts blockers, allowing Roy Williams to make the tackle on Marvin Harrison.
Result of the play, ref awards a TD to the Colts. No clue on how it is scored statwise, whether Williams gets credit for the tackle, Harrison for the TD, or for full yardage to the endzone, or just to where he was tackled. But that's an example of how there can be a tackle and a TD awarded on a play.
A Touchdown Awarded (Palpably Unfair Act) 1. When Referee determines a palpably unfair act deprived a team of a touchdown. (Example: Player comes off bench and tackles runner apparently en route to touchdown.)
First, in this situation the referee would not be able to determine if the act deprived a team of a touchdown because the player was tackled by an eligble player. Had the player been tackled by an ineligible player then the referee could determine that the team was deprived of a TD. But in this instance, because an eligible player still had an opportunity at a tackle, the unfair act did not deprive the team of a touchdown, therefore this penalty would not come into play.
Second, because in the modern era a penalty of this type has not happened, there is no proof that even if as in your hypo the referee awarded a TD, that the eligible tackler would receive credit for the tackle.