I can see you're gonna have this course wrapped up in a few weeks! The lap joint in the second pic looks as though the weld could penetrate the bottom plate a bit more. When I did my test piece (umpteen times) I struggled to get the toe of the weld to cut in to the bottom plate sufficiently and it would be 'undercut'. I increased the power and then it burnt away the edge of the top plate. I found that the best way was to move the arc slightly onto the bottom plate rather than straight into the corner, if you see what I mean. I have since heard though that my tutor was particularly fussy, which I suppose is a good thing really.
neat welding. Perhaps a little under powered or possibly wire feed slightly too high, as the welds are 'sitting up' a bit. It's difficult to tell from the photos but have you tried putting one piece in a vice and hammering the other? If it moves within a few whacks then it'll be becasue you're not be getting enough penetration.
The lower weld on the top picture looks to have a gap between it and the top plate.
The brown stain is usually caused by gas-flow too low, or it's being blown around slightly.
It's good going though, two out of three look good! It's just a case of trying various settings and practicing so you know if something's not right as soon as you start a weld and you have a chance to correct it before doing the whole run.