"Sushi rice" doesn't come preseasoned.
All ingredients added to "sushi rice" is done so AFTER it is done cooking. Pretty much you steam it, let it chill, then add all the junk in. Technically speaking, all of the stuff was probably originally a way to preserve the rice so that it would last longer, but the flavor of vinegar is not too desirable, so sweetening became popular (that's my guess). Now a days, white rice vinegar has a very clear and clean and crisp flavor that doesn't require much, if any, sugar to compensate. Again, a dilution of sake with white rice vinegar will usually do the same thing. If someone is marketing 'sushi rice', it's probably semi-gimmicky. It may be a very good strain of rice for sushi, but that it is ultimately white rice.
I'm not sure about the 'boiling a mix' thing- haven't heard of that. But... y'know what they say, every Japanese family has their own secrets. Heck, if you google recipes for "sushi rice," you'll get bombarded by at least a thousand different mixes and fancy techniques to get different but similar results. I recommend going the simplest most holistic route, but ultimately it just depends on your taste prefence. Some cool tricks result in neat flavors. I think I'm going to try your boiling thing using some kombu and benito flakes.
The stickiness is definitely dependent upon the water/rice ratio, cooking temperature, and the amount of water allowed to escape during steaming. A pressure cooker will probably hold more liquid than a saucepan, while a rice cooker will not lose any liquid. Rice cookers will give you consistent results, so, while I don't use one, I would actually recommend getting used to the settings so that you can get what you want. Even if the rice isn't sticky as it comes out, a small amount of added liquid (the alcohol) will fluff up the rice nicely if its mixed in properly. Technically speaking, you're 'supposed' to let the rice cool to room temperature.
I don't know any ratios or measurements (in terms of added liquid/sugar), but the basics are 1.5c water to 1c White Rice and 2c water to 1c brown rice (yea- you can do brown rice sushi style). I typically do a max of 1/8c liquid per 1c dry rice, but rarely ever use that much liquid. It sounds ridiculous, but even the humidity can have an effect on stickiness, so a specific recipe will never give you exactly what you specifically want. Baking, for example, is CRAAAZY complicated when you consider the conditions of the bakery. Room temp and humidity can cause the 2/5 ratio for bread baking to fluctuate to as much as 2.75/5- which... technically speaking, is a pretty huge difference (which is why a rice cooker is nice for its consistency, but how the rice rests will partially depend upon the temp/humidity). Ultimately, it's about getting a desirable texture, stickiness and fluffiness. All of that will depend upon how it is steamed and the quantity of alcoholic liquid you add afterwards, other ingredients don't really play into this factor, so long as you don't add too much to disrupt the giri of each rice (thai basil is a great additive for rice- after it's done steaming, just toss some thai basil in there while it does it rests, the result is a nice fragrance and flavor).
*shrugs* The fun of cooking is experimentation, as long as you have an idea of what variables you're dealing with, you can eventually get something palatable. I never really took sushi recipes seriously since most of them are ridiculous and gimmicky. Again, it's all derived from vinegar being used as a perservative. So as long as you keep that in mind, regulate the liquid content with the steaming method, you'll find something you'll like. And, well, if it tastes like crap, you can either adjust your added liquid (again, nigiri-sake and mizurin blend is nice) or throw some sugar in. I would DEFINITELY stabilize a BASE before getting comfortable adding extra ingredients (like seaweed).
For extra fun- use arborio rice. fricken amazing for sushi. I had some dry left over after making risotto, that stuff blows the pants off of any white rice I've ever used.
Right- I'm no expert, but I'd call myself a follower of Gourmandism. So- I definitely recommend experimenting and having fun and just thinking about the whole process.