The term "double bevel" can be ambiguous. When Phaedrus uses it, he -- like a lot of the other people -- means the knife is sharpened on both sides. You used it the same way I do.
A double bevel -- the way we mean it -- can do wonders for lots of knives; but isn't that big a deal or doesn't matter at all with others.
It's not easy to do freehand unless you're a very good sharpener or are doing the cutting bevel as a fairly obtuse micro-bevel. Micro-beveling has about as much probability to dull as to work, on the other hand, a lot of people end up accidentally deburring. In any case, new freehanders should focus on consistency and getting the flattest bevels possible.
If you're using some sort of "precision" system like an EP, you can do double bevels very easily. It just means sharpening the knife twice.
Sometimes people "thin" their knives at a fairly acute angle, then sharpen over it at a more durable, usable angle as a way to prevent wedging. For knives which are ground thick, or thick at the heel, that's not a bad idea at all. But thinning requires moving a lot of material and you really ought to wait until you're a very consistent sharpener. Like other coarse stone work, it's not a "just jump in," area of sharpening.
Don't worry, you didn't ruin anything. It's a helluva lot easier to add a double bevel than to take one out.
Flattening means just what it sounds like. Stones dish from use. Also, stones are frequently shipped with convex surfaces. In any case, you can't really keep a consistent angle if the surface of the stone is anything but flat. Stones tend to chip along their edges and at the corners because of the lack of support there and the way the forces of sharpening are transmitted. Counter-intuitively, they also tend to develop rails along the edges. All eight edges should be chamfered before the stone is used (or ASAP, anyway), along with all four corners. You can do it at a 45* angle if you like. Chamfering acts like a buttress, moving the vector of stress to a stronger part of the stone.
A nagura can't flatten, it's too soft. The purpose of a nagura is primarily to speed the production of "mud," and secondarily to do some minor lapping. As to lapping, I think rubbing the benchstones against one another does a faster, better job. You do have to be careful not to let them stick together.
As your flattening tool, I recommend 3M Drywall Screen (available from Home Depot, Lowe's, etc.) if you're trying to save money -- two packs should last decades. If you don't care about expense, get a DMT XXC.
Worth mentioning that stones should well be soaked and everything should be very wet when you flatten.
If you want to know the symmetry, look at the bevels and compare their widths. In order to see them clearly, use the Magic Marker Trick (MMT). The MMT will show you a lot of other things too -- like whether you're sharpening at all, whether you're developing high and low spots, etc.
Don't worry about overly technical ratios like 70/30. Your knives were shipped with a symmetry born of cheapness, not calculation. The maximum asymmetry which can still be productively maintained with a rod will have one side's bevel twice as long as the other's (actually pretty darn close to 70/30).
Start by heavily marking the entire length of the edges on both sides of the knife with marker. Start sharpening on the dominant side of the knife, and sharpen until you can just feel the burr. Turn the knife over, and chase the burr until you can just feel it on the second side. Now compare the bevels' widths. As you learn to sharpen, it's more likely that you'll see all sorts of horrible flaws which its more important to correct than any given symmetry.
For the time being, you'll end sticking fairly close to what the maker ground on because that's the way the knife is going to burr anyway as long as you start each new grit with the dominant side of the knife. It makes mathematical/physics/engineering sense if you have some idea of what's going on -- but would take forever to explain at this stage.
Note: Hold the knife by the handle, point out, edge down. If you're sharpening right-handed asymmetry, the right side of the knife is "dominant." Left-handed asymmetry, the left side is dominant.