It isn't the case that there are two Republican defectors.
Rather, the Republican Caucus understands that this is a contentious vote that might hit legislators hard come election time. Because the Caucus has two more people than it needs to pass DeVos, the Caucus strategically selected two Republicans to vote no so that they can be spared the backlash come election time. It is extremely unlikely that a third Republican will vote "no" - this is all planned, and this is very normal.
Yep. That's why the vote isn't "one Republican away" and why Collins and Murkowski deserve no credit. If they really wanted to take a stand, either one of them could've stopped DeVos from making it out of committee.