RE - rx

The trick I learned about who and whom - many years ago

This is how I was taught (granted it was over 50 years ago), but for what it is worth – unless someone else has a more updated way of distinguishing the two.  

I think your confusion is that you are recasting too much of the sentence trying to make the hint work.

Your example:  "The patient is a 65-year-old lady who/whom we have seen in our office." 

To use the “he, she, we, they”  or the  “him, her, us, them”  trick, you only look at a few words following who/whom

(In case anyone is confused as to which goes with which:

him, her, us, them = him and them end in M = whom)

So, you would look at it this way – which sounds correct:

We have seen HER/HIM

or

We have seen SHE/HE

I would be interested if any English majors out there have a different trick.

Thanks.