Hey Sarah, sounds like you are doing fine and Noah is doing well.
Claire used to be like that too, with the screaming if she was left alone. What I would do was give her something to hold onto (usually her teething ring or a small soft toy) and sing to her. As soon as I sing twinkle twinkle, she's all smiles. She also like music, so I play her music. If I aam leaving her on the floor, I make sure she can still see and hear me, and I talk to her. Some days she just needs cuddles though. She is a cuddly baby. Do you have a sling or baby bjorn or similar?
Dont stress too much about the milestones, as the others have said.
Claire didnt discover her feet till about 5 months. One day she wasnt interested at all, nd the next day she just wouldnt keep them out of her mouth. The first time she rolled from tummy to back was at three months but then she only did it 4 - 5 times over the followinf two months. She definately wasnt sitting up at 4 months. She has only really strted to sit on her own in the last week but she does still flop over.