Afaik, unless it's a really posh engagement, men only do seiza for the first few minutes, until the toast is over, and then go into crosslegged style.  Was your experience different?

males would usually sit cross-legged and females do that mermaid thing a lot. Honestly I can do either of those for 6 hours fine... But try that for a week straight and you have problems...no?

yes sir

I practiced tea ceremony for a year. It still hurts like hell doing 正座.
So yeah, sit properly for 10 min and then go cross legged. That's my strategy anyway...

You do eventually get just about as good at it as anyone else in Japan.  I lived in a all tatami house for two years (with no real furniture) which helped a lot, and two and a half additional years in Japan.  Another thing is that I just have a bad back (and I'm 26!), so I switch to seiza all the time when my back is hurting, even when I'm sitting in my desk chair at my apartmet.  I just do seiza on top of the chair.

I can sit seiza for longer than most of my students but not quite as long as most adults.  The tea ceremony club does an event at cultural festival where any student or teacher can come do the ceremony, and a lot of my students give up half way but I can make it through a full ceremony with little trouble.