The instructions aren't clear
Ain't that an understatement! Even 7 years later, they're still misleading, or confusing at best.
IMO, and from what I've read, adding orange flesh and juice to a fermentation yields disappointing results. What makes oranges so yummy are the sugars that accompany the aromas and flavors. Once those sugars ferment out, all that's left is a vile, sour flavor. Also much of those wonderful flavors and aromas the fruit offers are driven off when added to the fermentation. Flavor and aroma compounds may change when going through fermentation, and AFAIK, not always for the better.
Usually when adding citrus fruit to a beer, we limit it by only adding the zest (the thin outer color part of the skin), omitting the pith (white skin, underneath the zest) as well as the fruit pulp (flesh and juice) itself. The "recipe" and "instructions" don't even mention adding the zest, which for beer brewers is usually the most valuable part of the fruit.
A few years ago I added whole grapefruit peels, both zest and pith to the whirlpool of a Saison wort, with surprising and excellent results. Aside from flavor compounds, the added bitterness from the pith added a dimension that made it interesting and arguably better. It was well liked during our Guild's yearly picnic in the hot sun.
I'd say:
Add the zest to either the whirlpool, at flameout, or at 5'. Optionally, add some to the fermenter together with the dry hops, pre-steeped in vodka or just by themselves. May want to bag them with a few marbles so they don't just float.
Then eat and enjoy the juicy fruit underneath the peel! Win-win!
Perhaps some others can chime in on this M-80 recipe and how to use the Valencia Oranges.