Re: A nail is close to sidewall, repair? replace? or just let it be?
I had a similar thing on my previous car with 9k on the tires. The right thing to do is have it professionally plugged and patched from the inside ... but unfortunately, the tire manufacturer's association has put out guidelines to all the tire shops that they should not repair if the puncture is in the area of the first row of tread blocks (the shoulder blocks). The rational is the repair must be in the "flat" part of the profile.
This is a particularly arbitrary guideline on high performance tires with especially wide shoulder blocks ... but I digress.
I could not find any of the chain shops that would patch it and I did not want to drop $200 plus on a tire, so I ended up plugging it myself and have had no issues, but that is a risk I chose to take.
You might have no choice but to replace it.
The bling wagon: 2014 Grand Cherokee Limited, 5.7, RA4, Lux2, OA2 sporting LTX A/Ts
The mileage slave: 2010 Jetta Sportwagen TDI