FED refers to a runner who has just scored as a retired runner, and if you're going to call INT on one, you get either the runner who could have been put out, or, if you're not sure which that would be, the runner closest to home:
Rule 8-4-2g:
If a retired runner interferes, and in the judgment of the umpire,
another runner could have been put out, the umpire shall declare that runner
out. If the umpire is uncertain who would have been played on, the runner
closest to home shall be called out;
However, I would NOT have INT here, since interference with a thrown ball must be intentional (per the same rule). It's not easy to get hit in the back on purpose. And trying and failing to get out of the way would not make it intentional. That judgment is HTBT, however.
The retired runner is not required to "disappear" upon scoring, and may legally remove a bat.