It will be possible to summon Atomos without using Astral Flow.
In regards to the absorption effect, it is the same effect as that of dark magic spell Absorb-Attri and the blue magic spell Voracious Trunk. The effect duration from the monster you absorb an effect from will carry over, so if you absorb it when there are only a few seconds left, it will only remain on your party members for this duration as well.
Essentially, when it comes to Blood Pacts, Atomos is similar to Odin and Alexander in the sense that it is summoned just for this ability alone; however, Astral Flow is not required. There are no plans to add other Blood Pact abilities for Atomos.