Well first, since people have always been able to heal with gems already in the pre-fight screen, whether or not one can from the barracks is fairly irrelevant. I would assume the "grey-out" on other units when one is being produced is because one can only "produce" one unit at a time. Previously when one unit was building all other units were hidden from view. Now, since all units are always visible, they still can't click to build two at a time. Thus all others are greyed. Healing has never had anything to do with the buildings at all.
@Prinza the Hunter I can deal with them being pushy with the diamonds. I prefer it to be available but not pushy, but that isn't what bothers me. Having a button that is regularly pressed and then sometimes swapping that button with a diamond spending button is akin to bait-and-switch. It is something that causes people to spend diamonds when they did not mean to. Being pushy to try to encourage spending is fair game. Taking people's diamonds because they are tricked should not be. Confirmation dialog on spending diamonds or not, an accidental double click instead of single or a little bit of lag that makes a player think zero clicks have been done yet and so a second is made can happen too easily as it is. Or if one has the confirmation turned off.
Same is true with that cancel button. Most times I open a barracks there is no button at all because no one is hurt. Then when someone dies I intentionally go to my barracks and a button appears to recruit a replacement. I typically do not click on a building when it is in the process of recruiting. So, habit is, if I see a button below a troop icon I click it. It's a standard pattern recognition/response that humans do by nature. The fact that the buttons have different labels or icons doesn't not matter (yes, they are different and so there is a probability of noticing the difference in time or not). Heck, I know I've canceled productions in supplies buildings on accident by this very thing (slightly different as that's a different building opens then the one I thought I was clicking on, which is also a problem that will occur here, but same principal).