I've gone through MoI a few more times than Garryl it seems, but I mostly agree with that assessment though I have some things to add.
1: Soulmelds can't simply be turned off then on at will since the rules for shaping and binding them are pretty clear it's something you can usually do only once a day. Sure they could be unshaped, but that looks like it means you can't benefit from it for the rest of the day until you meet the criteria for being able to shape soulmelds again. Also note that even if a meld doesn't have any essentia invested in it, it is still quite noticeable.
2: There seem to be no rules for covering them up or otherwise disguising them. There probably should be though given certain situations, so it's worth it to ask a DM about. Some of the melds are not only noticeable, they also give away alignment.
3:Garryl hit it spot on as far as I can tell. Soulmelds might occupy a slot like an item, but are not themselves items and thus they would work in wild shape and such, provided the creature has the equivalent slots to actually have the melds bound in the first place. Check MoI page 169 for details about specific slots, and the differences between chakras and item slots for various creatures.