> But is it technically legitimate to have "$alternate-tier/option[]" in
> the second xsl:when block ?

well its's syntactically correct, but as you haven't shown your input or
the definition of $alternate-tier, or said what output you want, no one
other than you can say if it does what you want it to do.

> Looks like if similar node exists in parallel tree and matches same
> conditions, it sets it's value to "N/A".

No XSLT engine should ever generate the string "N/A" automatically.
The string value of the special double value not-a-number is "NaN" so
you sometimes see that in unexpected places, but never  "N/A".

David