Hi
You are right that
https://www.ghisler.com/950_beta.htm should be updated to run
instead of
in offline install step 4. The .inf file was changed in beta 2.
On the other hand, right now
https://www.ghisler.ch/install/beta/ (linked from the download page) contains also beta 2 cab files. Try to download them and run the installer again.
As for the unattended install, I don't have much experience with that. But the tcmain.inf seems to be rather readable so you should be able to modify what you need. Especially, if you have done silent TC installations in the past. And if not, maybe other members here can help.
Though, I'm not sure, why you need an unattended installation of beta TC in the first place. As far as I understand, the final version will have the classic installer.