You have a copper fill on the bottom, you need a ground fill. The copper fill fills all the copper unused by traces but doesn’t connect to any net (such as ground). So while there is a copper fill there, it isn’t touching any traces and is entirely isolated (it is not ground either). The ground fill will cause the fill to attach to any grounds it finds, but I’m unclear on where you put them. I’m hoping someone familiar with ground fills will answer. You could just experiment with ground fills or search for ground fill in the forum search tab as it has come up before several times.
Yes, but you need the ground seeds to create the ground fill so you do need to use them.
edit:
This forum article may help:
Peter