I totally prewash, because I wear a lot of cotton, denim and knits and all those things shrink like the dickens. It's bad enough buying something that is unwearable after washing, never mind spending the time to MAKE it....gaah.
I have often wondered with special occasion fabrics if I REALLY need to prewash -- brocade that I'm going to dry-clean for instance. Does it matter?
I have dyed white fabric with tea and coffee, but here are the problems:
1) You would need to BOIL the jeans with the coffee on the stovetop, which means you need a really big pot, a really strong person to lift it, and to keep stirring the jeans the whole time. You also need to not be afraid of shrinking the jeans!
2) The color has to be set in a vinegar bath (I forget the exact proportions but you can google them easily under "tea dyeing"). The smell tends to stick to clothing for quite a few washings.
3) The color will wash out gradually, so its hardly worth the bother.
I recommend finding some brown dye and doing a washing machine dye job (I'm not sure exactly how but again, the directions are all over the place online including around here).
Yes, you can use an invisible zipper instead (it will probably look better). Just disregard the zipper instructions and put in the invisible one however you prefer.
As for shortening, does the skirt have a lengthen/shorten line on the pattern? If so, then it would probably be best shortened at that line -- fold the pattern to make it shorter then remove the excess at the side. If the dress line is straight across, you can just shorten it however you want. If the line is curved, you will most likely have a harder time getting the hem to hang right after shortening. However, I have shortened curved hems by carefully pressing up the excess -- up to 3 inches, measured as accurately as possible with a seam gauge -- and chopping it off at the fold line. I don't necessarily recommend this method, but I have used it in the past.
It might be the order of instructions. I couldn't sew properly until I started ignoring the written instructions that came with the patterns. Honestly, many of them are counter-intuitive and don't allow for fitting as you go, which is my preferred method of sewing. Once I started thinking of the pattern shapes as templates, everything went together a lot better.
What I've done in the past when making pleated skirts in heavy or difficult fabric is to make the pleats in strips and the underlayer in a lighter fabric. You could use plain black cotton or plaid or something. That way you are not trying to fold and manipulate the more obnoxious top layer (and pleats definitely need to be topstitched at the fold or they will totally come undone).
I've made stuff too big, once or twice too small, and then I've put together things that were just.....wtf? They didn't fit, period. It doesn't help that I'm flat as a board up top with HUGE shoulders (some XL womens shirts are too small, just through the shoulders), a long torso, and then curves for miles below the waist.
Skirts and pants are easier for me than dresses and tops but yeah, everything has to be fitted specially. The finished garment measurements are a big boon to me as well.
Burda has one in their Young section that is pretty nice (worked for me), easy and form-fitting. I forgot the number but it comes in cropped pullover or regular zip-up styles. I'm probably going to do the pullover one for summer with short sleeves.
It needs to be interfaced, probably with something relatively stiff. There may also be bust pads in the dress to give it shape. I would suggest using some interfacing, iron-on is fine if the dress is cotton but otherwise you need the sew-in stuff. I'm not sure what else, since you didn't post a pic so its hard to know exactly what the issue is.