I've seen lots of replaced valve seats fail after 5, 10k miles or more. They don't always fail immediately if they've been incorrectly fitted.
Maybe you intended this statement as a joke, but I'd say that by definition it's impossible for a properly installed seat to drop. If it drops then it wasn't properly installed.
Exactly. All you have to do is install them right. I just asked around before I installed my first set. It's been a while now but I bet I have installed at least 50 sets of them. As far as I know I have never had one that I installed come loose. I do think the seat floor is a big part of the job and that a lot of people ignore it. I have pulled many a seat and you can SEE where the exhaust was getting under the seat. Of course, then you need to stock taller as well as bigger OD seats. Of course, that was then because I have never had my own machine shop. I have never even had my own TIG welder!
Well she is all back together and running great , idle is about perfect . Trouble is I can now really hear that bloody timing chain now the topends are so quiet !! Maybe I should double the valve gap to hide it ??