it sounds like you are trying to add or change a detail map for a hero, which as far as i know isn't possible, though i could be wrong. incase it isn't clear, mask 1 red controls how intensely the detail mask appears, but isn't the detail mask itself and can't modify the detail mask. i would guess the detail mask is more or less built into each hero's individual shader and can't be adjusted.
again, i could be wrong on this, but consider the conflict of having an item with a custom detail mask for a hero with its own detail mask, used on both the basemesh and possibly other items. a custom detail map for one item or set would cause problems with the base model and other items which might be equipped at the same time. even on a hero with no detail map, this would again cause problems if other people decided to make items with their own detail maps.

if anyone has a more informed answer i'd be glad to hear it, but i think you're out of luck if you're looking to actually implement a custom detail map for your workshop item. (unless, of course, you are just looking to mod your own client, in which case the possibilities are closer to endless)

even if decimate worked with uvs (some paid software does it) i highly recommend doing this manually anyway.
the idea is to get the best possible representative geometry of your hi/mid model with the least amount of tris, which really can't be done by any algorithm (..yet?)

it's more of an art than a calculation; as helljumper recommends, get rid of anything you don't need, anything you won't see, and simplify anything that's not particularly obvious or which can't be approximated with normal or other masks.

also, keep in mind most people will only ever see your model from the in-game overhead view, so it's only critically important for your model to look really good from that particular angle.

to answer your question most directly, would YOU accept your one sided cape? :)

i agree with akaize's solution regarding a lower-resolution backside, especially if you have no better option and few tris to spare. i'd also keep in mind rigging/weight mapping and the positions of valve's preexisting bones when creating the lower resolution model (assuming drow's model has bones for her cape) so as to avoid any possible artifacts or glitching from one side of the cape clipping with the other side during animations. definitely check it out in-game to make sure you dont have any major problems with this.
(note: a lot of items seem to have been allowed in game with -minor- issues like this, but dont use that as an example :)

as far as uv's and textures go, i'd try to allow at least a small space for the back-facing section if you arent able to mirror the uvs with the main side. despite what akaize recommends, i suggest NEVER using a flat color/texutre for any area of your mesh, unless you are absolutely sure nobody will ever possibly see it. also remember in dota 2, every color texture space also correlates to normal, mask1 and mask2 texture spaces.

as if i haven't already written enough -- i'd ignore lod0 as far as dota2 items are concerned. the lod0 will never be seen except in the portrait box ingame. if you are working on an item that will be particularly obvious in the hero's portrait, it may be worth considering, but also be aware it'll use the same exact textures as the lod1. for almost every item, a lod0 is essentially useless and probably a waste of time.

-in short, yes, you do need seperate geometry for thin sections like capes that can be seen from both sides. all valve's dota2 hero shaders (as far as i know) do not support or render back-facing polygons :P
(FOR SOME REASON, THEY DO RENDER THE INTERIOR OF TREES; check that one out yourself - lol)

i'm sure this was more information than you were looking for, but i hope some of it helps!

yeah, that unwrap looks good.
and also yes, you usually want the seam placement as inconspicous as possible because you're likely to get artifacts from baking along the seams. often there will be some glitches along seams in normal maps for example, although there are a few tricks for dealing with this if you have to have a seam where it might be too noticeable.

transparency is allowed (for some heroes at least; some individual hero shaders may not allow it) but only full (100%) transparency, ie only values of 0 or 255 in alpha of the color mask are used.

gyrocopter, for example, has full-transparent sections in the wings and part of the cap. a number of heroes with things like leaves (treant protector, prophet's treants) use transparency to achieve more detail. helljumper is right though, as far as i know, dota's shaders dont allow semi-transparency. i also recall tvidotto mentioning this while streaming; semi-transparency isn't possible without modifying the shaders, which i don't think is allowed.

anyway, for reference, to enable transparency in blender, assign a material to the geometry you want to manipulate, enable the 'transparency' box in the material settings and set the alpha value to something less than 1.
if you want to use an image map with an alpha channel to make only certain areas transparent, make sure 'z transparency' mode is selected and set the alpha value to 0. (this makes the material fully transparent without an image assigned to influence its alpha value)
assign your image map (with a valid alpha channel) to the material, making sure 'use alpha' is enabled, and scroll down to the map's influence settings. in the diffuse section, along with 'color,' which is enabled by default, check 'alpha' and make sure its value is set to 1. this should give your map's alpha channel control of the transparency of the material.

hope that helps!

to make a seam, just select the edges you want to split the uvs along, and click 'mark seam' in the shading/uvs panel (in edit mode). then unwrap the geometry normally; it will automatically split the uvs along the seam.
i would place the seam lengthwise along the bottom of your antennae, or whichever side will be seen least.

if you're new to seams, i would try experimenting with different seam placements and see what works best, just to get the feel of it. just remember you usually want to place seams in the least visible parts of your model.

if you're looking for a video tutorial with for better explanation of seams or general uv unwrapping, you'll find plenty with a quick search on youtube :)

have you tried using vertex groups or weight painting to assign each bracer to its respective bone?