timing chain replacement
#3
I wouldn't tackle replacing the timing chain if you're not too mechanically inclined. Why do you want to replace it? But if you're going to tear into the side of the engine, go ahead and replace the tensioners before they give you issues.
#4
Why not?
Why would you not want to replace the chain? If you got it all open, then replace everything that could possibly wear out going forward. Doing so removes more risk of future failures. The Nissan TSB states to replace the secondary (short) cam drive chains along with the nylon tensioner shoes. Tensioner replacement not required. But if those chains have been riding on the steel tensioner piston, i.e. metal on metal, why would you not put out a few extra dollars and replace them? The main chain usually does not hit metal on metal if the main slack guide and tensioner fail as they typically do. If you have torn this far into an engine, do it right and replace all parts that could pose future risk.