Newt Gingrich was a lifelong republican, but he stopped being conservative in this century, and only recently started again. When he entered the race, a large majority here at FR were uninterested in his campaign, seeing him as the guy who sat on the couch with Pelosi, among other less-than-conservative activities.
Tim Pawlenty was a lifelong republican, and a conservative, although some here argued otherwise. He was boring, but he's pro-life, pro-guns, cut taxes as governor, opposed Obamacare. OnTheIssues rates him a hard-core conservative, although some people dismiss that group.
Michelle Bachmann was a lifelong conservative. Though she won the Iowa straw poll, she was targeted by the pro-Palin crowd, first for comments made by Ed Rollins, and then because she wasn't deferential enough to Palin.
Rick Perry must be one of your "former democrats", although that was over 20 years ago; he was also a conservative republican, but because of his position on his own state paying tuition for children of illegals, and his gardasil issue, he was attacked by some as a fake conservative (again, OnTheIssues has him a hard-core conservative, and Palin had called him a true conservative leader). So he was attacked, especially again by Palin supporters who saw him as a hindrance to getting her into the race. Then he self-destructed at the debates, so it didn't really matter.
Who knows what Herman Cain was. He apparently voted for democrats for president at one time, but since he never held any elected office, it's hard to pin down what party he would govern as. Many considered him a solid conservative, but probably not a life-time republican (maybe he is to you, like I said, I'm interested in your list). He was ignored until Palin said she wasn't running, then the palin folks panicked and jumped on the Cain Train, only to suffer dissappointment when he quit over baseless sexual harrassment charges.
Rick Santorum seems to be a lifetime republican and a conservative, but some here just hated him because they thought he was stopping Newt Gingrich.
Ron Paul isn't a "life-long republican", but he was never a democrat. Probably one of the ones you called libertarian; I presume you were also including Gary Johnson (solid former governor).
Buddy Roemer is another one of the democrat-switchers, and was never a conservative, but he also never registered in any primaries, so he hardly drew votes from anybody else.
Mitt Romney never ran as a democrat, but he wasn't a conservative either.
Thad McCotter was a conservative lifelong republican, but nobody seemed to know he was running.
Jon Huntsman was a lifelong republican, but not particularly conservative. But if anything, he stole votes from Romney; hardly a GOP-e trick to water down the conservative vote.
So, let's see. You said there were 16 candidates registered for the nomination. Here's the list as I can find it:
Rick Perry, Rick Santorum, Newt Gingrich, Michelle Bachmann, Herman Cain -- The five who were considered conservative here at FR by a reasonable number of people, at some point in time.
Thad McCotter -- definitely conservative
Tim Pawlenty -- A conservative as well.
Ron Paul -- Some consider him a conservative.
I see 5 to 8 people who were fighting for the conservative vote. Plus Sarah Palin, the non-candidate who had strong support, supporters who attacked other conservatives to keep the field open for her.
I can't even fathom how ringers make Romney look good. Most people couldn't name 8 of the candidates without looking them up -- they were non-entities. The conservatives were getting all the attention.
I can go through all of them and the only ones with a clear cut active political life as Republicans and Conservatives over the course of their lives were ~ trumpets blare ~ GIngrich, Santorum and Caine.
All the others were faulty. Romney probably had an advisor who did the math and realized he needed as many ringers in the circle as he could get!