For the purposes of adding a card or attack to your momentum by the means of ending a turn in the end phase, can your oponent use the R on Absurd Strength? Does adding cards to your momentum in the end phase by clearing the card pool count as card effects? I think not but just clarifying.

...Absurd Strength specifies "during the combat phase", before any other considerations - the timing is completely wrong.

But no, attacks going to your momentum during the end phase is a game rule, not a card effect.