Nice looking bike you got there, ha, cleaner than mine at the moment. My battery does drop 5% or more initially, more so if it's been resting after a full charge and quite normal I think. If I am pushing the bike hard it can also drop in big chunks, then recover somewhat after I take a break. I'm used to this from riding my Bafang BBS01 so doesn't concern me.
I am not sure why my range seems lower, but I like to ride faster so I think that is a factor. Thing's aren't so flat here either, so maybe there is more up and down than I realize. But lately I'm just not worrying about it as I don't need the range for my 20 mile errands and I still have 50% battery to throttle it up the steep 700' hill and get home with 20% left. I will probably get a spare eventually and that might highlight if my original battery is lacking, but I don't think so.
As far as rattles, I have always had some intermittent noises and couldn't figure out where they were coming from. I think the cranks might rattle a little. I notice it most when I go over bumps. I suppose it could also be the front and/or rear suspension, although my front suspension bikes don't make these noises. The battery doesn't seem to be loose and it doesn't make rattle when I shake it.