Well, I have to admit that I don't use those batch files myself. Looking at TCMDHERE.BTM, I suspect that that registry setting only sets the current directory for Take Command itself and only when it is first launched. I don't think it will affect new tabs in an existing instance of TC.
I'm attaching my own shell-integration batch file, which takes a different approach to opening new tabs. It also adds a few more shell verbs for .BTM files, including Edit, Print, and Debug. Try it; I think it may do what you want.