Hi Andrew, and thank you for asking these questions. I hope you know you're not alone in asking them, and I am thinking over them as I read. I don't have great answers, but sometimes "talking" about it this way helps get closer to one.
This specific question is one I've wondered about too. I have read opinions by others more influential, wise, and intelligent than me who have (gently and not-so-gently) pointed out the unfortunate degree to which Christians have allowed fear to influence us -- whether in our relationship with politics, education, community-building, "foreign policy," etc. and I think it can be true of science too.
Even though we're told in 2 Timothy 1:7 that "God has not given us a spirit of fear, but of power and of love and of a sound mind," we are still trained in fear from birth sometimes, and it is hard to overcome.
So what I hear you asking is how people who are "regressive" in that way can still do so much good. And I also ask myself that question because I formerly held a rather antagonistic view of science (and am still working through what I actually believe), so I can also ask myself, did I do any good as a YEC? Or did I do good back when I held to fear-based political views that alienated others? Most of the people I love still hold some of these views, and I know they do good -- I am not better than them.
I suppose it all comes back to what God can do, not what any of us can do. He can bring about good through anyone, even people who are completely evil. If "the good Christianity can cause" is rooted in God's work, and not ours, then he deserves the credit. As Rich Mullins once said (quoting a teacher of his): "God spoke to Balaam through his [donkey], and He has been speaking through [donkeys] ever since. So, if God should choose to speak through you, you need not think too highly of yourself."
I don't mean any of this as an excuse for being "regressive." I guess for many of us it can be a very long process, and I'm grateful that God is more patient than I am.