I’ll answer your last questions first and then give you my thoughts on the age-worthiness of your wine.
RS (residual sugar) “Dry” (no sugar remaining) is usually considered 0.2% or less (2 g/L), so your wine looks like it is classified as dry. For my wines, I usually look for 0.1% or less residual sugar to be double sure. You don’t want any opportunistic spoilage yeast or bacteria munching on any leftover sugar as the wine is aging or, heaven forbid, when it’s bottled. If a re-fermentation happens when you don’t want it to, you risk off-aromas, turbidity, and elevated carbon dioxide levels.
Molecular SO2, conceptually, is a way of interpreting the sulfur dioxide content of a wine using the wine’s pH as context. The more acidic a wine is (and the lower its pH), the more effective any sulfur dioxide addition is as an anti-microbial agent. Free SO2 has three distinct species: SO32- (sulfite ions), HSO2– (bisulfite ions), and molecular SO2. Molecular SO2 is the form that kills or inhibits microbes and the proportion of molecular SO2 for any given measurement of free SO2 rises as pH drops (more acidic). The more acidic your wine, the more effective any sulfur dioxide addition will be, and vice versa.
The molecular SO2 “gold standard” level, where scientists feel comfortable stating that “no microbes are going to live in this wine” is 0.8 mg/L (or ppm). If you know your wine’s pH and FSO2, you can look up the molecular SO2 in correlation charts on the internet. 0.8 mg/L is quite a high standard to meet, and to be honest, most winemakers I know never try to achieve it. Let me explain why: Your wine has a pH of 3.57, so to achieve 0.8 mg/L molecular SO2, you’d have to make sure your wine had about 47 ppm FSO2. That’s quite a lot, much higher than what I tend to see as the average for red wines, say, around 27–32 ppm FSO2 at bottling. Imagine how much FSO2 you’d have to add to your wine if you had a more typical red wine pH like 3.75–3.85, which isn’t uncommon in the popular ready-to-drink red wine styles made in the in the US today. To achieve 0.8 mg/L molecular SO2 at a pH of 3.80, you’d have to make sure you were measuring 79 ppm FSO2, which is so high that you’d be able to smell it in your wine – in fact, you’d probably be able to smell the SO2 as you approached the glass from across the room! This is why, with sulfur dioxide additions, most winemakers try to forge a balance between microbially “safe and sane” but also sensorially-acceptable. For a well-rounded approach to sulfur dioxide management and anti-microbial safety, try a combination of factors, including keeping pH’s reasonable (it’s OK to add tartaric acid in order to lower pH), exclude oxygen during aging, practice good sanitation, and keep FSO2 levels above 25 ppm but not excessively high.
Looking at your provided numbers, it looks like your total SO2 is already very high. The commercial legal limit in the US is 350 mg/L and you’re at 250 mg/L already. At 250 mg/L, you could be at a point where that amount of SO2 will negatively impact the flavors and aromas. In lighter reds, like Pinot Noir, high levels of SO2 can actually start to bleach color. Your FSO2 is 37 mg/L, which I’d also call plenty high. My advice is to be careful with any further additions; you wouldn’t want to make an addition until or unless the wine dropped below 25 or even 20 (typical levels). If I were you, I’d bottle this wine up now since it’s already been in the barrel for 28 months and then would let it develop in bottle.
The ageability of red wines depends on so many things. It’s important to remember that when wines age, a number of chemical reactions happen over time. Oxygen present in the wine upon bottling will continue to interact with the wine’s components, oxidizing colors from blue and purple hues into reds and oranges. Aroma and flavor compounds in red wines will change over time, trending in general from fresh, fruitier flavors (like cherry, strawberry, and rhubarb on a Pinot Noir, for example) to what are called “bottle bouquet” or tertiary aromas and flavors like mushroom, tea, tobacco, and leather. Smaller tannin and polyphenolic molecules will condense together to form larger structures, which often fall out of solution to form the sediment common on the sides of older bottles. This is actually the reason that wine tends to taste smoother and less rough as it ages. Your tongue can actually sense the smaller particles very acutely; large polyphenolic complexes just don’t register on your tongue’s receptors in the same way because many of them are too big to bind.
Molecular SO2, conceptually, is a way of interpreting the sulfur dioxide content of a wine using the wine’s pH as context.
You list quite a few of the parameters (VA, pH, etc.), which can contribute to good ageability. It’s important the wine be dry so there is no danger of spontaneous fermentation in the bottle created by leaving a food source for bacteria and yeast. Having enough acid in the wine is critical as well as wines are more prone to oxidation and microbial degradation the higher the pH (lower acidity). Having enough alcohol is critical as well as it serves as a natural preservative in addition to being a key factor supporting the wine and its components after a lot of the primary “fresh” fruit flavors and aromas are gone or diminished. The tannin and color levels in the wine will also contribute to a longer lifespan for a wine. These elements are powerful natural antioxidants and as such, the more tannic and extracted your wine, the more “guts” it will have to see it through the long run. In your case, the pH and TA look good, you’ve got a dry wine and enough alcohol so I’d say unless it’s really orange and oxidized your prognosis should be good for peaking in 2 years, tasting great for the next 7 years or so. Not knowing any more details about how the wine was made and aged it’s impossible to predict perfectly, but if solid winemakering techniques were employed, it seems like you’re on the right track to an age-worthy Bordeaux-style blend.