Uncle Joker said:
It seems that Willas only change the way of how to calculate the total STR, it does not change the STR of any character at all.
The STR of HL or HM character is not decreased to Zero.
What Willas is doing is changing which characters are eligible
to be included in the calculation of total challenge STR. So he is working directly on the character, not the challenge, by placing a play restriction on it (as opposed to changing some characteristic of the character).
So, since Willas is working at the character level, "immune to character abilities" would allow a character to ignore the "doesn't count its STR" play restriction that Willas tries to place on it. Immunity would let that Lanni/Martell character count its STR during the challenge.