How important is the Republican National Convention for the nomination process?

Absolutely unimportant. It didn't used to be that way. In the current environment, delegates are "pledged" to vote for a particular candidate at the convention. If John Smith and Steve Jones are running for president as Republicans and you're a delegate who's pledged to vote for Jones, that's who you vote for. But in olden times, delegates weren't pledged; if someone wanted to vote for Bear Bryant (the football coach) they could, and someone did. Back then conventions were FUN! Now they're just big pep rallies.