Yes, and it is graded gluten free here. Modern diluted soju is usually made from tapioca or sweet potato in the first place, so wheat never enters the picture. Even the traditional styles distilled from wheat or barley end up gluten free, because distillation carries alcohol and volatile aroma across and leaves the grain protein behind in the still. The thing to check is what gets added after distilling, particularly in flavoured bottlings.

It depends which soju. Traditional soju is distilled from a rice, barley or wheat mash and tastes of it. The mainstream green bottle version is diluted soju: high proof neutral spirit column distilled from sweet potato, tapioca or other starch, watered down to drinking strength and finished with sweeteners and a little citric acid. The 1965 ban on distilling rice is what pushed the whole category in that direction, and although the ban was lifted in 1999, the cheap diluted style is still what most people drink.

Usually yes, in the mass market bottles. Saccharin was standard for decades, and modern labels use combinations of stevia, sucralose, acesulfame K, erythritol, sorbitol, xylitol or plain sugar. If sugar alcohols upset your gut, this is the detail that matters more than the alcohol, and it varies brand by brand and even between a brand core bottle and its flavoured line. Traditional pot distilled soju is typically unsweetened.

On the trigger grades, yes. Soju is graded gluten free with moderate histamine and tyramine and only borderline sulfite. Beer grades high for gluten, histamine, tyramine and sulfite all at once, and wine grades high for histamine, sulfite and salicylate. The trade off is dose and additives: soju is stronger than either, is drunk in rounds, and carries sweeteners that beer and wine do not.