If you saw The Martian, you might remember the arguments among NASA managers about how much information on Watney's predicament should be revealed to the public. It was pointed out that their only legal option was to lay it all out there. That's pretty much how government agencies actually work, and it's not a bad thing.
Government officials, exercising their office, do not have "rights" from their office, they have "powers". Its important to distinguish between those concepts.
Smith has the power to issue subpoenas, and Congress has the power to impose consequences if it believes that valid subpoenas from its officers have been denied (there are some options for judicial enforcement as well, but they are both less likely to be legally available and more practically difficult to apply if they are available against executive officers, so are mostly irrelevant.)
I don't believe the good congressman Smith is actually trying to get people killed. There is no indication he's making a good faith effort to understand their findings, he's looking to justify his beliefs. And that's ok, i guess. The good people of texas have a lot riding on oil being safe to burn. They should do everything they can to keep that oil flowing. But they shouldn't be endangering the researchers specifically.
The only acceptable risk to impose on the scientists, is the same risk we all face collectively. We collectively need to make decisions about climate change based on NOAA's findings. Don't shoot the messenger.
Not necessarily -- they might be, ianal, but it's not the case that all government e-mails are automatically part of the public record. I recall from back in my journalism days that government workers have some assurance of confidentiality w/r/t e-mail communications if that assurance is needed to effectively carry out their job.