This thread is confusing for me. Ikkyo is a finishing technique...If you're that far along in your execution where there is resistance then that means your atemi has failed you...On the practice mat displays of resistance not withstanding....you should not "counter-resist" anything if someone is non-compliant... either change the execution of technique or soften them up with a few well placed "taps" BEFORE applying Ikkyo.
PS. To the experienced Aikidoka and those who know Aiki...people who try to "out muscle" you have already submitted...they just don't know it yet. The answer is never to "out muscle" them back.
Practice Practice Practice.