They may not want to debate with Trump on live TV, but I think they risk serious damage to their campaigns if they don't. Besides, FOX would lose many, many viewers if the leader in the polls isn't present because the other candidates fear him.
I think you are right, except, if the GOP could get him excluded from the debates, then his numbers could or would be excluded from the polls, by collusion among the pollsters. IOW, say Trump gets 25% and Jeb gets 25% and Walker gets 25 and the rest of them get single digits. Well, the pollsters would just grade on the curve and cease mentioning Trump. I can’t see that being anything effective, but they are stupid enough to try.
Trump is basically playing “Occupy the Republican Party”, LOL. I think it’s great.