Will it put me in danger to talk further with [name withheld] who has a large and growing following on the Internet to share what I have been told about alien plans, to help him perhaps with a warning, or would that be fruitless and only cause a backlash for me?

In effect, the warning has been given. You have told him already your perspective, done out of the goodness of your heart without a first concern about yourself and your safety and the vulnerability in sharing what you know with someone involved with high-level government officials who you know are often untrustworthy. So the cat is out of the bag already. If you wish to get back in touch and cement this in place with a final answer as to your willingness to collaborate with him, then this will be a second opportunity for him to hear your position and with somewhat greater clarity and conviction, having considered his offer to you, and this will get more attention. The unfortunate reality here is he is already heavily corrupted by mind control manipulation and will likely be unable to hear your message and give it unbiased consideration. You will be talking to deaf ears and you will see this happen again and again with members of the Secret Space Program and others who are pro-extraterrestrial in their willingness to trust and give them support and will look askance at any who doubt or raise anything negative about them. This will put you in the spotlight yet again, but you are in the spotlight every time you go forth with an interview that is posted on the Internet. So this would be no different. The eyes are always watching and roaming to see who is doing what.