OK well after a trip to the Apple store where a technicial took the laptop to the back room for quite a long time (half an hour at least) - the issue with the wireless has been repaired. He reported that indeed there was an antenna connection that was loose which he re-seated. Now I have the same full quality connection throughout my house that I had before the problem.
While I was at the store I made a point to compare my new Samsung display with the rMBP display models they had in store. Interestingly all three of the display models were LG displays, and all three of them had a bluer white point than my new Samsung has. I am not going to say that they are more correct, but they were certainly 'whiter' (i.e. bluer), and the Samsung yellower. One of the LG displays had clear and obvious image retention, just as bad as the LG display that I had replaced (i.e. noticeable image retention within 1 minute of holding a static image on screen); but the other two LG displays did not show any image retention after many minutes of leaving a static image up (at least 4 minutes).
I tried selecting different color profiles and doing the "color calibration" that is available via the normal display preferences and I could either select the bluest setting which was much bluer than the LGs on display, or the next setting down which is the default and yellower than the LGs. I have not used any other software that would let me adjust the color calibration with finer control. And I don't really know how to evaluate black levels so I can't say if they are better or worse with this Samsung than the LG.
I guess I would like a bluer white point with this Samsung since it does look slightly yellow-ish to me, but it's not the biggest deal in the world for me, so I'll just live with it or see if there is any way to do a more sophisticated color calibration than the stock settings available from System Preferences.
Also, I do not notice any non-uniformity in the color of this Samsung.
And finally - comparing the Samsung and LG side-by-side, I was clearly imagining things when I thought that the pixel grid was more visible on the Samsung. The pixel grid and "painted-on" aspect of the fine retina display was exactly identical between the Samsung and LGs.