It's hard to read through all of these stories without aching in pain for all that's been experienced.
I don't think lying is ever the right choice. Talking about things in an age appropriate way is the healthiest approach.
I don't know your story but I'm sure it's similar enough to what we've all experienced.
So here is the truth: people hurt people. We do it every day with our actions, intentionally and unintentionally. Each person is responsible for their own actions. In our family, we are working on making good choices and not hurting each other. We talk about our feelings and we take responsibility when we hurt someone. If we hurt someone deliberately, there's a consequence for that.
when kids are little, the full details are irrelevant. You can keep it pretty generic. If you stay in a situation where there's active abuse or adultery happening, then that's going to have more damaging effects on your daughter than leaving and showing her what a healthy relationship looks like. Remember that she's going to pick the kind of relationship she sees modeled in her own life.
I know it's so hard right now. But don't focus on what the "story" should be. Go with age appropriate information and make healthy choices for your daughter since she's dependant on you for that.