I do like the idea Devil, but like Philbaaa pointed out, people could abuse that system without extra limitations to it. I don't think a time limit to lock something makes sense, as you can't determine how far away a robber camp might be, and offense players might very well be able to take several camps at once as the first senders with actual amounts needed for the camps. Nothing wrong with that in my book. Possbily a troop amount limit could work, but sometimes especially near the end of a game, people might send lower amounts of offense which they don't mind being wiped out, as they want to use them up and need the stolen goods to crank out def. I did that myself recently at the end of a server. So I'm not sure that is the best solution either.
However, I fully agree that the incoming waves need to be shown on robber camps on the mobile app. I could say a lot more about the mobile app, but if this one thing in particular would be changed on it, it would make so many people's lives much easier. And, if that is fixed, then the only other excuse that people have for undercutting on robber camps is if multiple people send at the same time on a robber camp. I believe it takes 30 sec (on speed server, maybe more on normal server) for that attack to show up on the camp. I'd suggest having an attack show up immediately on a camp. Fix those two things, and there would be no excuse for undercutting...and then people who do, will be found out, and kingdoms can deal with members not following rules/etiquette. But I've gone through the trouble of asking others on mobile which camps are open, and it is a pain...so please fix the mobile app to allow us to see incoming attacks on camps.