I'm definitely using the correct size plug. I have two adaptors I use, one at home and one at work. The one at work is the one that was supplied with my original Mk2. The one I use at home came recommended in this thread and a few others. Both adaptors have been in use for quite some time, are the exact same size, and require the same amount of "effort" to insert and remove (i.e. not much at all.) No other adaptors other than these two have been used at any time. So that rules out the wrong plug.

As to being "heavy handed"... The geometry of the plug doesn't really allow you to put any excess pressure on the leaf switch itself. Lateral movements of the plug while connected will probably break the adaptor plug off of the board or something, but because the plug fits snugly into the socket, it seems pretty impossible to break the leaf switch with lateral movement (unless the adaptor was halfway-inserted and you used the plug as a "lever" to put excess pressure directly on the leaf switch.) So the only real-world scenario that it seems could break the switch is, as you said, a larger adaptor, and that's not the case.

In any event I can assure you I am careful when inserting the plug, especially having sent one back for this problem before. However the switch has still failed. I can see the switch in there but it's hard to tell if it's making contact, it must be, or I wouldn't be having this problem. So unless there was a way for me to "bend back" the leaf, it sounds like I'm sending this thing back for repairs. I'm just frustrated that a flimsy leaf switch was chosen to detect AC or DC power, there MUST be a more robust solution.