Yeah well i mean nash is really going to be optimal vs a very good player, it's kind of a defensive play that helps so you cannot be exploited. I find in turbo SNG's it's easier to just stick to the chart, but against some opponants as you mention, i will open up, and against some, i may go tighter than the chart suggests (mostly for calling ranges vs particular players). SO yeah it's really just a guide, and if your facing off vs a very good player, i suggest you just stick to the chart. If your both playing nash, and your shoving wider than it suggests, you are now being exploited.
As for that other mumbo jumbo, if you ever want to add tables, it's probably adviseable to not look so much into your heads up game, stick to the nash, and of course adjust to opponants who are not playing optimally.
But yes as you mention also, i rarely ship a 20BB stack in preflop heads up, i will generally try to raise, but it's also unusual to be at 20 BB stacks in turbos, but anything near 15bb's i will generally start shipping it.
You really wouldnt want to be shoving too much wider than it suggests at 20bb's, it's just risking too much, when we have more optimal ways to play.
Anyways, im babbling and making little sense, so im going to shutup now.