I guess what I really want to know is foolproof glueing secrets. Like using fabric tabs to glue fabric to a barrette. Does hot glue ever hold? how long does gem tac last if washed?
Different kinds of glue are best for different kinds of materials, and for different kinds of stress (direction of stress, water/washing stress, etc). Knowing which glue to use for what, and what it can and can't do is a whole art/craft in itself.
Here are a few answers to some of your questions though:
....I'm unfamiliar with the tabs technique but it could work in situations where there isn't enough contact between fabric item and metal item, etc....you can try various kinds of "mechanical hold"** along with the adhesive hold, or alone, and the tabs idea could give you something partly similar.
isn't all that strong for some things but holds fabric surprisingly well, though very stiff. It doesn't hold that well on smooth surfaces though (like metal/plastic) although in those situations a mechanical hold can help if one can be worked out. (Glue gun glues can be used along with other glues though if it's applied to different areas than the other glues just before joining...that way it will act as nails to hold the parts together but the other glue will be the real adhesive once it dries/cures.)
...For fabrics, various of the stronger permanent "white glues"
will be good (and washable), and Jewel-It or Gem Tac are made especially to hold metals to fabric. Weldbond and Crafter's Pick The Ultimate are also two strong permanent white glues.
Acrylic paint (and "fabric paint" which is more flexible because it's had clear textile medium added to it) is basically the same thing and should hold well.
Don't wash any of those glues until they've had at least a week to also "cure" but they're all "washable (if you're worried, put items in a mesh bag or wash by hand).
...For metals and many other materials, silicone type glues
are commonly used (but need to be applied as instructed for the best hold); E6000 is one strong one of that type.
...Two-part epoxy glues
are very strong, though they aren't at all flexible. (The longer the setting time, the stronger the glue).
...Gorilla Glue and other polyurethane glues
will bond anything to anything, but are yellow and swell while curing so must be weighted/clamped and have any oozes removed.
(cyanoacrylate glues, superglues, etc) are very good for a few things and lousy for other things. The parts must match exactly, for example, and not generally be porous; and the stress they're good for is a pulling-apart stress not stress from the side which is much more common.
** A "mechanical" hold, as opposed to an adhesive hold, is when there is something physically holding something else (no glue, adhesive, etc, necessary). That would often mean something wrapped around something else (e.g., thin wire wrapped around the layers of a hairbow while or after being attached to a French barrette finding), or screws or nails, or even a glue that was allowed to pass through/around/under something so that it would have a physical grip on the area it was attaching to (e.g., if a glue got in/around/under some part of a metal barrette or any area that's dimensional or has undercuts or edges, etc).
You can read more about various types of glues on this page at my site along with links there to other pages, though the rest of the site is primarly about polymer clay:http://glassattic.com/polymer/glues-Diluent.htm
And this page has a lot too on the various types of "glue":http://en.wikipedia.org/wiki/List_of_glues
How do you open the bottle of E6000 after you have glued it shut?
I don't think I've seen E6000 in a bottle, but you can put a bit of Vaseline on the threads of its tube, or you can put a piece of plastic wrap between the cap and the tube before screwing on.
If I haven't done that and can't get the cap off I'll just poke a new hole in the tube with a pointed toothpick and squeeze the glue through the hole. The tube eventually gets kind of holey, but each hole will seal itself off.