You're thinking too hard dude. We don't know that they are missing a digestive system; what we do know is that even with their sole purpose to apparently be eating people, they never actually finish digesting them, and instead either poop out or vomit rainbow corpses. With what we know now, more then likely titans vomit out humans and actually digest shifters in order to try and turn back to normal (all of this done subconsciously). The type of juices in the stomach are irrelevant, like I said you're thinking too much. If it makes you feel better, your stomach doesn't actually have as much "juice" in it as was in the titans stomach that Eren was in; it was all probably for dramatic effect and even then it's still irrelevant.
They obviously didn't know that Eren was the coordinate, evident by Bertolt and Reiner being hella surprised when Eren accidentally sics the areas titans on Reiner. As for the Ymir thing, let me remind you that Eren more then likely gained his abilities from his dad (whom he later ate), but didn't know about it for many years later. It's not far fetched to believe that the same happened to child Reiner, Bertolt, and friends; that is, by chance a titan ate one of the kids and gained the ability to turn human again.
Also, I don't know what you mean by full titan. Why would any of them want to become a full titan? That means losing the ability to turn human again, and instead wandering the world with your only objective being to either eat people or kill other titans. Hardly desirable I'd say...lol.