>>relationships have a bargaining element?

Yes, generally understood to be true in psychology.

>>if we forgive, we fear it devalues or weakens our bargaining power

I can’t argue your personal experience, but I don’t think this one is major theory or something generally accepted to be true about forgiveness.

>>Why else are we so tempted in a debate to rejoin with “yeah, well you do <wrong thing> too

Because “whataboutism” is just a logical fallacy that people use as a common defensive tactic, but it’s not inherent to forgiveness.