If something has a resistance to, for example frost (nords), 100% ( therefore immune) - if I use a weakness to frost spell at strength 100 does this mean the nord would no longer be immune to frost for the duration of the weakness to frost?

If so, if a weapon has frost damage 30pts and weakness to frost 30pts does that mean that before the frost damage takes effect I'd have to hit a nord at least once for no elemental damage for the weakness effect to be applied?

Your first statement is yes, the weakness to frost spell would break his resistance to frost.

I believe that the spell of weakness to X and X damage works on same hit. I know it does on
a weapon I made that soul traps and absorbs health, and it does so on one strike. At least
put the weakness to X spell first in the line-up, if you know what I mean.