Hum, right. Now as you mention it.
But as i see it Epic follows the more detailed version of the Wh40k Codices. Or it is whishful thinking of me

But then i would only grant Greater Daemons
Fearless. I don't see the Lesser Daemons as so disciplined that they will be immune to hackdown hits.
So to summaries my thoughts about Summoning in Epic Wh40k style:
Replace a unit with a Character as casualty (but this generates no BM) and replace the unit with a Greater Daemon of the same faction.
Greater Daemons remains in play until destroyed.
For lesser Daemons: Buy Lesser Daemons as upgrades for formations (eg 3 Lesser Daemons per upgrade. Or more?). Icon Bearers* will be the visual marker to see which formations have Lesser Daemons waiting in the warp.
At the start of the formations activation place any number of (or better: all?) Lesser Daemons you have purchased for this formation in the same manner as in the current summoning rules.
The Lesser Daemons are now part of the formation and remain in play until destroyed.
So what do you think?
*Plus the Icon Bearer could grant
Inspiring ( for Khorne and Slaanesh) to the formation or
Invulnerable Save (for Tzeentch and Nurgle)to all units in the formation so long as the Icon Bearer remains in play.