Honestly, patience is the name of the game. As long as the decal isn't ripped or crumbled, you should be able to re-wet it and adjust it again.
Cut each decal out before you wet it, you should only need to dip it for a few seconds and then let the water do it's work. I usually use a damp q-tip to slide the decal from the sheet onto the part and to adjust it in place. Then, once you're sure it's in the right place use the dry side to barely kiss the decal to absorb the excess water on it. If you bump it off course, just use the wet end again to transfer a bit of water onto the decal and let it absorb so that it's re-positionable.
I personally don't use anything other than just water. No Mr Setter, Mr Softer, or topcoat. I do my models just decals and panel liner and they look great to me. But it's your model, do what looks good to you.