Without getting into the larger conservatives vs. conservatives argument, two points about your item on David Frum.
One is that it’s a bit over the top to say that Frum, aside from his other comments on conservatives, “smeared” Levin. It’s not clear from your post whether you knew this, but that Harper’s piece you mention was quoting from a long post Frum wrote on his website. If you read Frum’s original piece, his criticism was mostly directed at Andy McCarthy and Kathryn Jean Lopez, not Levin. And certainly he didn’t “smear” Levin in the sense that he wrote anything inaccurate.
The second is that the context in which Frum wrote his post is important: originally Jim Manzi (a conservative) attacked Levin on the Corner in response to accusations that conservatives are unable to think critically about where they get their ideas and news. By chastising Manzi for calling out Levin, McCarthy and Lopez confirmed those accusations, whether or not they were justified in the first place. So if you, not realizing this chain of events, now go after Frum for disagreeing with McCarthy and Lopez… well, we’re at risk of feeding right back into the idea that conservatives are not allowed to contradict other conservatives. Unless, of course, you reject the idea that Frum is a conservative entirely.