Sorry in advance for the novella;

Some things to think about when Gluing things together:

1) how heavy is your item and how much surface area are you gluing it to; If something is heavy but you're trying to glue it on a very narrow edge, its most likely NOT going to stay on, especially when moving about a convention.
1a) I would suggest moving to foam clay from dry clay; it is more expensive, but its much lighter, and works the same way, can adhere with contact cement (or a little bit of water and molding it onto EVA foam)

2) adhering to fabric is a whole different ball game, and yes, most of the time you're going to have to test. Some fabrics go to EVA foam easily with contact cement, however, you have to be SUPER careful when applying so it doesn't look weird (as sometimes the glue can show through and you can see if you have streaks/big globs of glue - for this reason I do not recommend hot glue when gluing large surface areas of fabric down to anything more rigid and flat)

3) never underestimate the power of hot glue. Yes it can make gross globs and strings, but any/most things on the insides of all my armor pieces are reinforced with huge globs of hot glue - especially straps, and especially around any body part that moves a lot naturally (underbust seams, legs, arms, etc.) Does the inside look CRAZY not at all aesthetic - Yes - But I do not care if it means my armor is not coming off while I walk around the convention .

4) for heavy things (or really anything) going onto fabric you're almost ALWAYS going to want to SEW down a base piece to the fabric; or the actual thing to the fabric. I recommend worbla for this, but I have seen people use foam. You can use some hot glue, fabric glue or contact cement to adhere the base piece to the fabric, then whipstitch around the edge, then use whatever glue works best for the material you sewed down and the material you're trying to glue down. For attaching things like Fabric straps for leg armor to EVA foam armor, I do use a small piece of eva foam on one end of the strap, sew it on with my sewing machine (do not recommend if you have delicate machine that can't sew through some heavy duty stuff - Also it will absolutely mean whatever needle you use is likely unfit for sewing straight fabric anymore - so reserve a couple of "dud" heavy duty needles for this) and then contact cement the EVA foam piece to the inside of the EVA foam armor.

PS your stuff looks great!