A Ph.D. in Science Education is a research degree, and people in that field are researchers. I would listen carefully to your adviser's concerns and your own gut feelings about sustained, independent research activity. In addition, PhDs in Science Education typically are involved in teacher education-- that is, preparing undergraduates to be K-12 science teachers-- and thus it is necessary to have K-12 science teaching experience yourself to be at all competitive for a university position. That experience is typically obtained before beginning the doctorate.
I recommend #1; the opportunity to pursue a doctorate in biology or any other field will still be there down the road, and in the meantime you will have gained experience and perspective.
Yes to all of the above. OP, I am in teacher education, and work and conduct research extensively with science educators.
You will not be employable in a College of Ed without at least three years of contracted secondary science teaching experience. They are other kinds of positions out there for science educators, including at zoos, wildlife parks, aquariums, and so on, but getting those is highly competitive, and typically requires a lot of experience as an intern or volunteer in those kinds of places. Many of these positions are funded by soft money, and require grantwriting expertise, too. There used to be more governmental positions for science educators, too, but those have been disappearing in federal and state budget cuts, and they never did pay very well.
The other thing to know about science education is that the research does not involve studying scientific phenomena, but rather conducting research into how to prepare K-12 teachers to be effective in the classroom. This is increasingly frustrating to many of my colleagues because, thanks to NCLB, we are seeing the marginalization of science and a serious turn away from the discovery-based process that comprises genuine science in our public school classrooms.
I don't mean to sound encouraging, and the world is certainly in need of good science educators. At the same time, I am seeing too many doctoral students in the sciences these days who think that a PhD in science ed will be the easier route to a postsecondary teaching position, and that is really
very unlikely.