I think MMR is 15-20% of the sporadic population and in ~100%...

I think MMR is 15-20% of the sporadic population and in ~100% for those with Lynch (which is about 5% of the total).
Usually chemo/radiation therapies are stopped when immunotherapies are used because there's little to no data on their interaction and tolerability.
Bearing in mind in nearly all cases immuniotherapies are used in trial stages or compassionate grounds.