You could add '5) did they never think of using grounder bone marrow?' Surely they're just as immune.

Honestly though at some point you have to just accept that a plot like this will have a few holes and choose to enjoy the show despite them. I like the show well enough to ignore everything I know about physiology, genetics and physics and just enjoy the drama. But to address each point:

1) Yeah it would have made more sense to send Raven and her little one man pod down with a geiger counter, but that wasn't the only reason they sent someone down. They were running out of air and they needed to decrease their population, fast. Sending down the 100 didn't just give them an experimental test group, it relieved some of their resource pressures. Also, there's going to be more to worry about than just radiation, so sending a group of settlers and seeing if they can hack it does make sense.

2) The 'instant radiation death' thing they have going on is totally unrealistic. If it was that bad, nothing would be alive. We're talking about ionizing radiation here - if it was strong enough to melt faces then it's strong enough for nothing to be immune to it. Resistance to radiation requires mechanisms to fix damaged DNA, that's not possible if the radiation is so intense that it causes organic matter to melt in seconds.

3) They probably had an idea - after all it's well known that astronauts (and pilots) are exposed to a higher level of radiation in space. However with no readings from the ground they'd have no idea whether or not the levels were comparable to the ones they were living with.

4) Yes. We're essentially talking about an extinction event here - those capable of dealing with the radiation would have bred, those that couldn't would not, and those that were heavily susceptible would have died in the first generation. There are numerous problems with the whole 'they evolved/ were genetically engineered to be resistant to radiation' idea. It's pretty much completely implausible, but hey, it's a good story.