In deciding to bottle, age or toss this batch, I suggest you spend some quality time with your barrel. Though you’re just past the usual bottling window (typical aging time for premium red wines is 10–18 months, depending on the varietal and style) you may be able to catch it before it goes south. Especially if the wine has been well taken care of, topped regularly, and sulfite added along the way, you should be fine. Though it sounds like you’ve been perhaps a little neglectful in your racking and sulfur regimen, you may be pleasantly surprised. Some wines do best when we walk away and let them do their own thing.
Thief yourself a little bit out of the barrel into a clean glass and go out into the sunlight. Give it a few good swirls and stick your nose in the glass. Do you like the way it smells? Does it still have some perceptible fruit? Is the wine free of oxidized and other aroma defects? Smells like Port, Sherry and nail polish can all be signs of spoilage. Because it’s only been racked once, I would watch out for aromas like burnt rubber, onion, or rotten eggs. Though rare for a wine at this age, there are some stinky compounds like mercaptans and sulfides that can live in deep in layers of undisturbed lees and infect an entire barrel. Since you’ve only racked once, you could be at risk.
Then take a look at the wine (inclining the glass over a sheet of white paper can help). Is the wine clear? Is the color still holding up? Though Nebbiolo can lend itself to age more into a brick red/garnet color spectrum, the color should at least be red still with maybe a little brick red around the edges. Last but not least, give it a taste. If it’s pleasant to you, there’s no reason not to bottle it. It probably doesn’t have monstrous tannins but you could always try egg white fining or a similar gentle fining method to smooth out any rough edges you may find.