I have done Little Hearts and Bigger Hearts. I do believe the science is light. We started off doing the science in the guide for a couple weeks but switched to Apologia instead because it just wasn't enough for my science loving gal. I also thought some of the experiments and activities were odd.
And yes, the discussion questions in the guide are usually open ended without any answers. There have been a few times where I wasn't sure what answer they were expecting to get.
As far as YE, I think there was only one reference I remember in the Bigger guide to skip if you aren't YE. I don't know about the other guides, but you wouldn't be skipping a bunch in the two guides I have done.
Hope that helps!