Perhaps the seminar is different, but I usually find more intensity and energy (=opportunity to strain) at seminars...
I just try to make sure especially with that Shihan that I get there early
There are others whose seminars I attend who do longer warm-ups, and that I like also, guess I'm flexible about stretching.
I'm not sure how I'd do if I stopped during class to stretch, then began again; no longer thirtysomething myself, I tend to stiffen up if I'm still for too long once I get going, guess it would depend on how long and just what was being stretched. I like stretching once warmed up, but if it was just one muscle group, the rest could get cold.