If I recall correctly the 1 1/8" Alpha Q cyclocross fork is tested to the same standards as their tandem fork. You should email their tech guys (response within 24h) to check same applies for 1".

Regarding fatigue I think it's unlikely that there's an issue. What you're feeling could be the difference between your carbon forked bike and a steel forked bike. Steel forks do flex and IMHO are no worse for it. I used to find the flex useful to gauge how hard I was braking as it helps avoid locking the wheel.

I find when I ride my non a-headset bikes the front end feels flexy. IMHO most of the flex comes from the spindly alloy stem rather than the fork, particularly when twisting the bars, so an aheadset system will help here.

I'm not sure upgrading the fork alone would be such a good idea. If you're not happy with a Steel Santana fork, where would you draw the line? Perhaps a better solution since you like tandeming would be to update the whole bike or frame and forks? Yes that does cost more but probably you would be happier in the end.